pgsql: DROP IF EXISTS for ROLE/USER/GROUP

Log Message:
-----------

DROP IF EXISTS for ROLE/USER/GROUP

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
drop_group.sgml (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_group.sgml.diff?r1=1.11&r2=1.12)
drop_role.sgml (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_role.sgml.diff?r1=1.1&r2=1.2)
drop_user.sgml (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_user.sgml.diff?r1=1.21&r2=1.22)
pgsql/src/backend/commands:
user.c (r1.167 -> r1.168)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/user.c.diff?r1=1.167&r2=1.168)
pgsql/src/backend/nodes:
copyfuncs.c (r1.325 -> r1.326)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c.diff?r1=1.325&r2=1.326)
equalfuncs.c (r1.261 -> r1.262)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/equalfuncs.c.diff?r1=1.261&r2=1.262)
pgsql/src/backend/parser:
gram.y (r2.525 -> r2.526)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.525&r2=2.526)
pgsql/src/include/nodes:
parsenodes.h (r1.299 -> r1.300)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/parsenodes.h.diff?r1=1.299&r2=1.300)
pgsql/src/test/regress/expected:
drop_if_exists.out (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/drop_if_exists.out.diff?r1=1.1&r2=1.2)
pgsql/src/test/regress/sql:
drop_if_exists.sql (r1.1 -> r1.2)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/drop_if_exists.sql.diff?r1=1.1&r2=1.2)