Improve tab completion for USER MAPPING

Hi all,

I found that the tab completion for USER MAPPING doesn't work fine.
For example,

The below is no problem.
postgres=# create user[TAB]
user              user mapping for

But this doesn't work fine. (Note that there is a white space between
'user' and [TAB])
postgres=# create user [TAB]
hoge_user          masahiko           pg_signal_backend

After manual input of the 'mapping', 'for' is added by tab completion.
It means that the tab completion for 'mapping' is not working.

Patch attached.
Please review it.

Regards,

--
Masahiko Sawada