Add PGETC (for pg_service.conf) and PGLOCALE (for locale dir)

Add PGETC (for pg_service.conf) and PGLOCALE (for locale dir)
environment variable processing to libpq.

The patch also adds code to our client apps so we set the environment
variable directly based on our binary location, unless it is already
set. This will allow our applications to emit proper locale messages
that are generated in libpq.