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.)