pgsql: Fix up tab completion for ROLEs and add some more completion

Log Message:
-----------
Fix up tab completion for ROLEs and add some more completion logic for
other stuff; change \du and \dg to be role-aware (Stefan Kaltenbrunner).
Also make tab completion fetch the list of GUC variables from pg_settings
instead of having a hard-wired copy of the list (Tom Lane).

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
psql-ref.sgml (r1.148 -> r1.149)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/psql-ref.sgml.diff?r1=1.148&r2=1.149)
pgsql/src/bin/psql:
command.c (r1.151 -> r1.152)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/command.c.diff?r1=1.151&r2=1.152)
describe.c (r1.122 -> r1.123)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/describe.c.diff?r1=1.122&r2=1.123)
describe.h (r1.28 -> r1.29)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/describe.h.diff?r1=1.28&r2=1.29)
tab-complete.c (r1.136 -> r1.137)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/tab-complete.c.diff?r1=1.136&r2=1.137)