Follow-up to 875314ed0bf3b

The code was duplicated in the various resolver backends.

Also, it was called after the call to `Curl_ipvalid`, which matters in
case of `CURLRES_IPV4` when called from `connect.c:bindlocal`. This
caused test 1048 to fail on classic MinGW.

The code ignores `conn->ip_version` as done previously in the
individual resolver backends.

Move the call to the `resolver_start` callback up to appease test 655,
which wants it to be called also for literal addresses.

Closes https://github.com/curl/curl/pull/4798

Also, use `CURLRES_IPV6` only for actual DNS resolution, not for IPv6
address support. This makes it possible to connect to IPv6 literals by
setting `ENABLE_IPV6` even without `getaddrinfo` support. It also fixes
the CMake build when using the synchronous resolver without
`getaddrinfo` support.

Closes https://github.com/curl/curl/pull/4662

The function does not return the same value as snprintf() normally does,
so readers may be mislead into thinking the code works differently than
it actually does. A different function name makes this easier to detect.

Reported-by: Tomas Hoger
Assisted-by: Daniel Gustafsson
Fixes #3296
Closes #3297

This avoids warnings about unused stuff.

Closes https://github.com/curl/curl/pull/2023

curl_printf.h defines printf to curl_mprintf, etc. This can cause
problems with external headers which may use
__attribute__((format(printf, ...))) markers etc.

To avoid that they cause problems with system includes, we include
curl_printf.h after any system headers. That makes the three last
headers to always be, and we keep them in this order:

 curl_printf.h
 curl_memory.h
 memdebug.h

None of them include system headers, they all do funny #defines.

Reported-by: David Benjamin

Fixes #743