Bumped next release version to become 7.23.0 for the changes

Slight rewording of the CURLOPT_URL SMTP sub-section.

Corrected the incorrect use of hyphens on the three uses of
"zero-terminated" with "zero terminated" to match the rest of the
document.

Corrected the use of an out of place hyphen in CURLOPT_NOPROXY section.

It seems some versions of the OpenSSL version defines don't come with L
appended to the number, so let's deal with that nicely.

With locking, plus test, plus documentation

Maximum amount of data a header callback is supposed to get in
a single call from libcurl is limited by the lowest value of
CURL_MAX_WRITE_SIZE and CURL_MAX_HTTP_HEADER.

When using Sun C compiler the preprocessor somehow inserts an extra space
in front of replaced symbol, breaking CURL_CHECK_DEF macro. To workaround
this, macro CURL_CHECK_DEF now ignores all leading whitespace in front of
symbol substitution result.

Removed the code that striped off the domain name when Curl_gethostname
returned the fully qualified domain name as the function has been
updated to return the un-qualified host name.

Replaced the use of HOSTNAME_MAX as the size of the buffer in the call
to Curl_gethostname with sizeof(host) as this is safer should the buffer
size ever be changed.

Moved HOSTNAME_MAX #define into curl_gethostname.h rather than being
locally defined in curl_gethostname.c, curl_ntlm_msgs.c and smtp.c.

Allow (*curl_write_callback) write callbacks to return
CURL_WRITEFUNC_OUT_OF_MEMORY to properly indicate libcurl of OOM conditions
inside the callback itself.

If a socket is larger than FD_SETSIZE, avoid using FD_SET() on the
platforms where this is possible.

Bug: http://curl.haxx.se/bug/view.cgi?id=3413274
Reported by: Tim Starling

When we use binary posts and regular ones intermixed on a single command
line, we cannot do strlen() etc on the data to figure out the length
(when inserting '&' and more). We must therefore keep track of the post
data length. Then we also end up setting the libcurl option with the
known size, so that we don't risk that libcurl will do strlen() on the
data.

This has the minor side-effect that --libcurl source codes now always
will use CURLOPT_POSTFIELDSIZE but I don't consider that terribly
damaging.

Bug: http://curl.haxx.se/bug/view.cgi?id=3413181
Reported by: Taneli Vähäkangas

Probably the wrong place, but I dont know better.

In some cases Content-Length: couldn't be replaced by an application

Also, indented some code properly

To avoid that the progress meter headers get output between each
transfer, make sure the bits gets kept when (re-)inited.

Reported by: Christopher Stone

my_setopt and my_setopt_str no longer ignores curl_easy_setopt result.

Fixed some OOM handling issues.