psql: initialize comment-begin setting to a useful value by default.

psql: initialize comment-begin setting to a useful value by default.

Readline's meta-# command is supposed to insert a comment marker
at the start of the current line.  However, the default marker is
"#" which is entirely unhelpful for SQL.  Set it to "-- " instead.
(This setting can still be overridden in one's ~/.inputrc file,
so this change won't affect people who have already taken steps
to make the command useful.)

Discussion: https://postgr.es/m/CAJcOf-cAdMVr7azeYR7nWKsNp7qhORzc84rV6d7m7knG5Hrtsw@mail.gmail.com