-J without -O shall not honor C-D filename

Previously it would use a 256 byte buffer and thus cut off very long
subject names. The limit is now upped to the receive buffer size, 16K.

Bug: http://curl.haxx.se/bug/view.cgi?id=3533045
Reported by: Anthony G. Basile

Bug: https://bugzilla.redhat.com/788526
Reported by: Enrico Scholz

forward declare the Configurable struct

Commit eeeba149 removed them and thus broke my Linux build

when calloc fails, return error! (Detected by Fortify)

Reported by: Robert B. Harris

Additionally, make hash checking ability mandatory in order to allow metalink
support in curl.

A command line option could be introduced to skip hash checking at runtime,
but the ability to check hashes should always be built-in when providing
metalink support.

Previously it could easily wrongly get repeated

The length/size options take longs so make sure to pass on such types.

Reported by: Neil Bowers
Bug: http://curl.haxx.se/mail/lib-2012-06/0001.html

Re-factored the smtp_state_*_resp() functions to 1) Match the constants
that were refactored in commit 00fddba6, 2) To be more readable and
3) To match their counterparties in pop3.c.

Verify that the "Saved to filename 'blabla'" message is only displayed when
the 'blabla' filename being used _actually_ has been specified by the server
in the Content-Disposition header.

Use relative path for unintended file creation postcheck.

Re-factored the SMTP_AUTH* constants, that are used by the state
machine, to be clearer to read.

The r7b had some bugs, and shouldnt be used.

These now detect incompleate header data and fail

Add a postcheck section to verify unintended file creation.

Remove needless <file> checks in verify section. Renumbering where appropriate.

When a <file> part is now specified with no contents at all, this
will actually verify that the specified file has no contents at all.
Previously file contents would be ignored.

Missed some comments that we identified during the SMTP tidy up earlier.

Corrected lines longer than 78 characters.

Removed unnecessary braces in smtp_state_helo_resp().

Introduced some comments in data sending functions.

Tidied up comments to match changes made in pop3.c.

For consistency with other SASL based functions renamed this function
to Curl_sasl_create_ntlm_type3_message() which better describes its
usage.

Corrected lines longer than 78 characters.

Changed POP3_AUTH_FINAL to POP3_AUTH to match SMTP code now that the
AUTH command is no longer sent on its own.

Introduced some comments in data sending functions.

Another attempt at trying to rational code and comment style.

After Steve's commit e336bc7c test 1319 and 1407 need to check
for CAPA instead of AUTH.

Added a service type parameter to Curl_sasl_create_digest_md5_message()
to allow the function to be used by different services rather than being
hard coded to "smtp".

Currently 1356 to 1362 succeed but a write failure is logged in traceNNNN.

Currently 1363 fails, so disabled for now.

Not all SASL enabled POP3 servers support the AUTH command on its own
when trying to detect the supported mechanisms. As such changed the
mechanism detection to use the CAPA command instead.