pgsql: Add some missing backslash commands to psql's tab-completion kno

Add some missing backslash commands to psql's tab-completion knowledge.

\if and related commands were overlooked here, as were \dRp and \dRs
from the logical-replication patch, as was \?.

While here, reformat the list to put each new first command letter on
a separate line; perhaps that will limit the need to reflow the whole
list when we add more commands in future.

Masahiko Sawada (reformatting by me)

Discussion: https://postgr.es/m/CAD21AoDW1QHtBsM33hV+Fg2mYEs+FWj4qtoCU72AwHAXQ3U6ZQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f1175556a17a193395326f45a3e595b4aa3a9eff

Modified Files
--------------
src/bin/psql/tab-complete.c | 30 ++++++++++++++++++++++--------
1 file changed, 22 insertions(+), 8 deletions(-)