Closes #750

As these two options provide identical functionality, the former for
SOCK5 proxies and the latter for HTTP proxies, merged the two options
together.

As such CURLOPT_SOCKS5_GSSAPI_SERVICE is marked as deprecated as of
7.49.0.

This value is set to TRUE or FALSE so should be a bool and not a long.

Following conversation on the libcurl mailing list.

...when GSS-API or Windows SSPI are not used.

Allow the service name to be overridden for DIGIST-MD5 and Kerberos 5
authentication in FTP, IMAP, POP3 and SMTP.

Calculate the service name and proxy service names locally, rather than
in url.c which will allow for us to support overriding the service name
for other protocols such as FTP, IMAP, POP3 and SMTP.

Closes #603

Closes #544

Closes #543

Closes #522

... as otherwise we might get stuck thinking there's no more data to
handle.

Reported-by: Damien Vielpeau

Fixes #737

mbedtls_ is the prefix used by the mbedTLS library itself so we should
avoid using that for our private functions.

This reverts commit 9c08b4f1.

Didn't help. Caused problems.

Fixes #756

Make (most) example snippets use the example.com domain instead of the
random ones picked and used before. Some of those were probably
legitimate sites and some not. example.com is designed for this purpose.

The space character after the status code is mandatory, even if the
reason phrase is empty (see RFC 7230 section 3.1.2)

Closes #755

Closes #754

Thinking it might help to apply patches etc with git.

It's a bad idea to send your passwords anywhere, especially over HTTP.
Modified example to send a picture instead.

Fixes #752

Now sorted into categories and organized in the same style we do the
TODO document. It will make each issue linked properly on the
https://curl.haxx.se/docs/knownbugs.html web page.

The sections should make it easier to find issues and issues related to
areas of the reader's specific interest.

Included a summary of the checksrc.bat updates and combined two krb5
changes as they should have been implemented at the same time.

Reported-by: Michael Osipov