The state machine should only be changed to IMAP_STARTTLS when the
STARTTLS command has been successfully sent to the server.

Updated the ssldone comment as multi mode is always used internally now.

This avoids witness message in output when running configure --help,
while sending the message to config.log for other configure runs.

Minor code tidy up to add comments similar to those used in the pop3
and imap end of resp functions, in order to assist anyone reading the
code and highlight the similarities between each of these protocols.

- version conditional overriding
- catch unexpanded XC macros
- fix double words in comments

Tor Arntsen analyzed and reported the issue.

URL: http://curl.haxx.se/mail/lib-2013-01/0306.html

m4 quadrigraph shell comment technique allows proper autoconf
parentheses balancing in shell 'case' statements. The presence
of unbalanced parentheses may otherwise trigger expansion bugs.

The most recent version of the SMTP RFC is RFC5321 and not RFC2821 as
previously documented.

Added RFC1870 and re-ordered list numerically.

smtp_state_upgrade_tls() would attempt to incorrectly complete the
upgrade to smtps and start the EHLO command if
Curl_ssl_connect_nonblocking() returned a failure code and if ssldone
was set to TRUE. This would only happen when a non-blocking API hadn't
been provided by the SSL implementation and curlssl_connect() was
called underneath.

pop3_state_upgrade_tls() would attempt to incorrectly complete the
upgrade to pop3s and start the CAPA command if
Curl_ssl_connect_nonblocking() returned a failure code and if ssldone
was set to TRUE. This would only happen when a non-blocking API hadn't
been provided by the SSL implementation and curlssl_connect() was
called underneath.

imap_state_upgrade_tls() would attempt to incorrectly complete the
upgrade to imaps and start the CAPABILITY command if
Curl_ssl_connect_nonblocking() returned a failure code and if ssldone
was set to TRUE. This would only happen when a non-blocking API hadn't
been provided by the SSL implementation and curlssl_connect() was
called underneath.

- Update comments
- Execute commands in subshells
- Faster path separator check
- Fix missing 'test' command
- Rename private macros
- Minimize AC_REQUIRE usage

Small tidy up to remove unnecessary return statements prior to the next
fix.

Some basic checks we make were placed early enough in generated
configure script when using autoconf 2.5X versions. Newer autoconf
versions expand these checks much further into the configure script,
rendering them useless. Using XC_CONFIGURE_PREAMBLE fixes placement
of early intended checks across all our autoconf supported versions.

The most recent version of the RFC is RFC4422 and not RFC2222 as
previously documented.

The most recent version of the SASL RFC is RFC4422 and not RFC2222 as
previously documented.

- document the double-quote and backslash need be escaped if quoting.
- libcurl formdata escape double-quote in filename by backslash.
- curl formparse can parse filename both contains '"' and ',' or ';'.
- curl now can uploading file with ',' or ';' in filename.

Bug: http://curl.haxx.se/bug/view.cgi?id=1171

Notes:

When running a configure script that has nested packages (for example
libcurl's configure with --enable-ares and c-ares sources embedded in
curl tree) and AC_CONFIG_SUBDIRS([nested-subdir]) machinery is used to
automatically run the nested configure script from within the parent
configure script, it happens that the nested _shell_ script will
inherit shell variables exported from the parent _shell_ script.

If for example parent configure script sets and exports LDFLAGS and LIBS
variables with proper values in order to link either a parent library or
program with a library which will be configured and built by a nested
package; It will happen that when the nested configure script runs, the
nested library does not exist yet and _any_ link-test done in the nested
configure will fail, such as those that autoconf macros perform in order
to detect existing compiler and its characteristics, the result is that
the nested configure script will fail with errors such as:

configure: error: C compiler cannot create executables

For now, we no longer export variables previously exported here.

On the other hand, AC_SUBST'ing them is appropriate and even with nested
packages each package's config.status gets its own package values.

So we reinstate AC_SUBST'ing previously AC_SUBST'ed variables.

Fix proper macro expansion order across autotools versions for
C compiler and preprocessor program checks.

Extended the fix from commit 8b15c84e to additionally exclude
pop3_state_apop_resp() if the CURL_DISABLE_CRYPTO_AUTH flag is
defined.

Bug: http://curl.haxx.se/mail/archive-2013-01/0017.html
Reported by: Ulrich Doehner

Reported by: Craig Davison
Bug: http://curl.haxx.se/mail/lib-2013-01/0234.html

Reported by: Craig Davison
Bug: http://curl.haxx.se/mail/lib-2013-01/0234.html

Corrected some function argument definitions to maximize the 80
character line length limit and be in keeping with the curl
coding style.

Fixed an issue where Curl_ssl_connect_nonblocking() wouldn't complete
correctly and the ssldone flag wouldn't be set to true for pop3s based
connections.

Bug introduced in commit: 4ffb8a63.

Added missing imap fix as per commit 709b3506.