Fix portability and safety issues in pqTraceFormatTimestamp.

Fix portability and safety issues in pqTraceFormatTimestamp.

Remove confusion between time_t and pg_time_t; neither
gettimeofday() nor localtime() deal in the latter.
libpq indeed has no business using <pgtime.h> at all.

Use snprintf not sprintf, to ensure we can't overrun the
supplied buffer.  (Unlikely, but let's be safe.)

Per buildfarm.