thread-safety: getpwuid_r()

Here is a patch to replace a getpwuid() call in the backend, for
thread-safety.

This is AFAICT the only call in the getpw*() family that needs to be
dealt with.

(There is also a getgrnam() call, but that is called very early in the
postmaster, before multiprocessing, so we can leave that as is.)

The solution here is actually quite attractive: We can replace the
getpwuid() call by the existing wrapper function pg_get_user_name(),
which internally uses getpwuid_r(). This also makes the code a bit
simpler. The same function is already used in libpq for a purpose that
mirrors the backend use, so it's also nicer to use the same function for
consistency.