pgsql: Properly update the 'group' flatfile when modifying the user, in

Log Message:
-----------
Properly update the 'group' flatfile when modifying the user, in case
they were added to a group.  Also fix visibility of our own changes when
creating the group file.  This fixes:

        test=> CREATE GROUP g1;
        CREATE GROUP

        test=> CREATE USER u1 IN GROUP g1;
        CREATE USER
        test=> \! cat /u/pg/data/global/pg_group
        "g1"    "u1"

        test=> CREATE USER u2 IN GROUP g1;
        CREATE USER
        test=> \! cat /u/pg/data/global/pg_group
        "g1"    "u1" "u2"

        test=> ALTER USER u2 RENAME TO u3;
        ALTER USER
        test=> \! cat /u/pg/data/global/pg_group
        "g1"    "u1" "u3"

[ this code does not exist in CVS head.]

Per report from Dennis Vshivkov

Tags:
----
REL8_0_STABLE

Modified Files:
--------------
    pgsql/src/backend/commands:
        user.c (r1.147 -> r1.147.4.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/user.c.diff?r1=1.147&r2=1.147.4.1)