Move init_ps_display from postgres.c to postmaster.c, putting it

Move init_ps_display from postgres.c to postmaster.c, putting it
just after receipt of the startup packet.  Now, postmaster children
that are waiting for client authentication response will show as
'postgres: user database host authentication'.  Also, do an
init_ps_display for startup/shutdown/checkpoint subprocesses,
so that they are readily identifiable as well.  Fix an obscure race
condition that could lead to Assert failure in the postmaster ---
attempting to start a checkpoint process before any connections have
been received led to calling PostmasterRandom before setting random_seed.