pgsql: Fix bug that allowed any logged-in user to SET ROLE to any other

Log Message:
-----------
Fix bug that allowed any logged-in user to SET ROLE to any other database user
id (CVE-2006-0553).  Also fix related bug in SET SESSION AUTHORIZATION that
allows unprivileged users to crash the server, if it has been compiled with
Asserts enabled.  The escalation-of-privilege risk exists only in 8.1.0-8.1.2.
However, the Assert-crash risk exists in all releases back to 7.3.
Thanks to Akio Ishida for reporting this problem.

Modified Files:
--------------
    pgsql/src/backend/commands:
        variable.c (r1.115 -> r1.116)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/variable.c.diff?r1=1.115&r2=1.116)
    pgsql/src/backend/utils/mb:
        encnames.c (r1.27 -> r1.28)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/encnames.c.diff?r1=1.27&r2=1.28)
    pgsql/src/backend/utils/misc:
        guc.c (r1.310 -> r1.311)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.310&r2=1.311)
    pgsql/src/include/utils:
        guc_tables.h (r1.20 -> r1.21)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/guc_tables.h.diff?r1=1.20&r2=1.21)