Make sure we don't call memcpy() if the argument is NULL even
though we also passed a zero length then, as the clang analyzer
whined and we want to limit warnings (even false positives) when
they're this easy to fix.

The change of (char) to (unsigned char) will fix long user names
and passwords on systems that have the char type signed by
default.

This function could only return CURLE_OK and by changing it to
a void instead, we can simplify code all over.

Simplified the code by removing a local variable completely.

This makes the code flow more obvious and reacts on the return
code properly, even if the code acted the same way before.

When multi interface is used, the SSL handshake is no longer
blocking when GnuTLS is used.

The recent overhaul of the SSL recv function made this treat a
zero returned from gnutls_record_recv() as an error, and this
caused our HTTPS test cases to fail. We leave it to upper layer
code to detect if an EOF is a problem or not.

No resolver anymore needs to use AI_CANONNAME and do reverse
lookups.  We should work hard to avoid having code that relies on
it.

This code would previously use dns_entry->addr->ai_canonname
instead of the given host name, which caused us grief and
problems since not all our resolver options do the reverse lookup
and I would also guess that it caused problems with KRB5/GSS with
virtual name-based hosts. Now the host name from the URL is used.

As reported in bug report #2987196, the code for ipv6 already did
the setting of this bit correctly so we copied that logic into
the Curl_ipv4_resolve_r() function as well. KRB code is the only
code we know that might need the cannonical name so only resolve
it for such requests!