pgsql: Add a GUC_SUPERUSER_ONLY flag to mark GUC variables that should

Log Message:
-----------
Add a GUC_SUPERUSER_ONLY flag to mark GUC variables that should not be
examinable by non-superusers, and use it to protect the recently-added
GUC variables for data directory and config files.  For now I have only
flagged those variables that could be used to deduce something about
the server's filesystem layout, but possibly we should also mark vars
related to logging settings and other admin-only information?

Modified Files:
--------------
    pgsql/src/backend/utils/misc:
        guc.c (r1.245 -> r1.246)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.245&r2=1.246)
    pgsql/src/include/utils:
        guc.h (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc.h.diff?r1=1.53&r2=1.54)
        guc_tables.h (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc_tables.h.diff?r1=1.17&r2=1.18)