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

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