Prevent continuing disk-space bloat when profiling (with PROFILE_PID_DIR

Prevent continuing disk-space bloat when profiling (with PROFILE_PID_DIR
enabled) and autovacuum is on.  Since there will be a steady stream of autovac
worker processes exiting and dropping gmon.out files, allowing them to make
separate subdirectories results in serious bloat; and it seems unlikely that
anyone will care about those profiles anyway.  Limit the damage by forcing all
autovac workers to dump in one subdirectory, PGDATA/gprof/avworker/.

Per report from Jšrg Beyer and subsequent discussion.