pgsql: Fix a couple of issues in recent patch to print updates to

Log Message:
-----------
Fix a couple of issues in recent patch to print updates to postgresql.conf
settings: avoid calling superuser() in contexts where it's not defined,
don't leak the transient copies of GetConfigOption output, and avoid the
whole exercise in postmaster child processes.

I found that actually no current caller of GetConfigOption has any use for
its internal check of GUC_SUPERUSER_ONLY.  But rather than just remove
that entirely, it seemed better to add a parameter indicating whether to
enforce the check.

Per report from Simon and subsequent testing.

Modified Files:
--------------
    pgsql/src/backend/utils/misc:
        guc-file.l (r1.61 -> r1.62)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc-file.l?r1=1.61&r2=1.62)
        guc.c (r1.519 -> r1.520)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.519&r2=1.520)
    pgsql/src/include/utils:
        guc.h (r1.105 -> r1.106)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc.h?r1=1.105&r2=1.106)
    pgsql/src/timezone:
        pgtz.c (r1.63 -> r1.64)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/timezone/pgtz.c?r1=1.63&r2=1.64)