... to please the new, slightly picker, checksrc.pl

Although this should never happen due to the relationship between the
'mech' and 'resp' variables, and the way they are allocated together,
it does cause problems for code analysis tools:

V595 The 'mech' pointer was utilized before it was verified against
     nullptr. Check lines: 376, 381. curl_sasl.c 376

Bug: https://github.com/curl/curl/issues/745
Reported-by: Alexis La Goutte

* Prefer dereference of string pointer rather than strlen()
* Free challenge pointer in one place
* Additional comments

* Prefer dereference of string pointer rather than strlen()
* Free challenge pointer in one place
* Additional comments

This wouldn't cause a problem because of the way the function is called,
but prior to this change, we were processing the challenge message when
the credentials were NULL rather than when the challenge message was
populated.

This also brings this part of the Kerberos 5 code in line with the
Negotiate code.

Although mutual authentication is currently turned off and can only be
enabled by changing libcurl source code, authentication using Kerberos
5 has been broken since commit 79543caf in this use case.

This wouldn't cause a problem because of the way the function is called,
but prior to this change, we were processing the challenge message when
the credentials were NULL rather than when the challenge message was
populated.

This also brings this part of the Kerberos 5 code in line with the
Negotiate code.

Prior to this change, we were generating the output token when the
credentials were NULL rather than when the output token was NULL.

This also brings this part of the Kerberos 5 code in line with the
Negotiate code.

Prior to this change, we were generating the SPN in the SSPI code when
the credentials were NULL and in the GSS-API code when the context was
empty. It is better to decouple the SPN generation from these checks
and only generate it when the SPN itself is NULL.

This also brings this part of the Kerberos 5 code in line with the
Negotiate code.

... checksrc of all test code is pending.

The quote scanner was too fragile, now look for a comma instead to find
the mode argument.

follow-up to 0326b067

sizeof(pointer) is no good for the buffer size!

Reported-by: Viktor Szakats

warning: implicit declaration of function 'sprintf_was_used'
         [-Wimplicit-function-declaration]

Follow up to the modications made to tests/libtest in commit 55452ebd
as we prefer not to use sprintf() now.

We never made a 7.25.1 release

It offers extra info from nghttp2 in certain error cases. Like for
example when trying prior-knowledge http2 on a server that doesn't speak
http2 at all. The error message is passed on as a verbose message to
libcurl.

Discussed in #722

The error callback was added in nghttp2 1.9.0

To be consistent with the Kerberos 5 context and other authentication
code.

For consistency with the spnego code.

For consistency with the spnego and oauth2 code moved the setting of
the host name outside of the Curl_auth_create_gssapi_user_messag()
function.

This will allow us to more easily override it in the future.

As this was introduced in the recent vauth changes and not a prior
release.

I had accidentally used the proxy server name for the host and the host
server name for the proxy in commit ad5e9bfd and 6d6f9ca1. Whilst
Windows SSPI was quite happy with this, GSS-API wasn't.

Thanks-to:  Michael Osipov