Change mingw-w64 printf format checks in public curl headers to use
__MINGW_PRINTF_FORMAT instead of gnu_printf. This syncs the format
checker with format string macros published via curl/system.h. (Also
disable format checks for mingw-w64 older than 3.0.0 (2013-09-20) and
classic-mingw, which do not support this macro.)

This fixes bogus format checker -Wformat warnings in 3rd party code
using curl format strings with the curl printf functions, when using
mingw-w64 7.0.0 (2019-11-10) and older (with GCC, MSVCRT).

It also allows to delete two workaounds for this within curl itself:

• setting -D__USE_MINGW_ANSI_STDIO=1 for mingw-w64 via cmake and
  configure for docs/examples and tests/http/clients.
  Ref: c730c85 build: make CURL_FORMAT_CURL_OFF_T[U] work with mingw-w64 <=7.0.0 #14640

The format check macro is incompatible (depending on mingw-w64 version
and configuration) with the C99 %z (size_t) format string used
internally by curl.

To work around this problem, override the format check style in curl
public headers to use gnu_printf. This is compatible with %z in all
mingw-w64 versions and allows keeping the C99 format strings internally.

Also:

• lib/ws.c: add missing space to an error message.
• docs/examples/ftpgetinfo.c: fix to use standard printf.

Ref: #14643 (take 1)
Follow-up to 3829759 #12489

• give public CURL_TEMP_PRINTF a better name? And/or its internal counterpart.