pgsql: Prevent pg_ctl from being run as root.

Log Message:
-----------
Prevent pg_ctl from being run as root.  Since it uses configuration files
owned by postgres, doing "pg_ctl start" as root could allow a privilege
escalation attack, as pointed out by iDEFENSE.  Of course the postmaster would
fail, but we ought to fail a little sooner to protect sysadmins unfamiliar
with Postgres.  The chosen fix is to disable root use of pg_ctl in all cases,
just to be confident there are no other holes.

Modified Files:
--------------
    pgsql/src/bin/pg_ctl:
        pg_ctl.c (r1.41 -> r1.42)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_ctl/pg_ctl.c.diff?r1=1.41&r2=1.42)