pgsql: Allow CREATE/ALTER ROLE PASSWORD NULL to allow restoring the

Log Message:
-----------
Allow CREATE/ALTER ROLE PASSWORD NULL to allow restoring the default state
of having no password.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        create_role.sgml (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_role.sgml.diff?r1=1.5&r2=1.6)
    pgsql/src/backend/commands:
        user.c (r1.166 -> r1.167)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/user.c.diff?r1=1.166&r2=1.167)
    pgsql/src/backend/parser:
        gram.y (r2.517 -> r2.518)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.517&r2=2.518)