thread-safety: gmtime_r(), localtime_r()

Here is a patch for using gmtime_r() and localtime_r() instead of
gmtime() and localtime(), for thread-safety.

There are a few affected calls in libpq and ecpg's libpgtypes, which are
probably effectively bugs, because those libraries already claim to be
thread-safe.

There is one affected call in the backend. Most of the backend
otherwise uses the custom functions pg_gmtime() and pg_localtime(),
which are implemented differently.

Some portability fun: gmtime_r() and localtime_r() are in POSIX but are
not available on Windows. Windows has functions gmtime_s() and
localtime_s() that can fulfill the same purpose, so we can add some
small wrappers around them. (Note that these *_s() functions are also
different from the *_s() functions in the bounds-checking extension of
C11. We are not using those here.)

MinGW exposes neither *_r() nor *_s() by default. You can get at the
POSIX-style *_r() functions by defining _POSIX_C_SOURCE appropriately
before including <time.h>. (There is apparently probably also a way to
get at the Windows-style *_s() functions by supplying some additional
options or defines. But we might as well just use the POSIX ones.)