errdetail_busy_db plural forms

I was looking for missing use of gettext plural forms, which led me to
errdetail_busy_db().

While we can't do much about this:

    errdetail("There are %d other session(s) and %d prepared transaction(s) using the database.",
              notherbackends, npreparedxacts);

I think it's still worth pluralizing the other cases

    errdetail("There are %d other session(s) using the database.",
              notherbackends);

and

    errdetail("There are %d prepared transaction(s) using the database.",
              npreparedxacts);

Especially the "other sessions" case is probably the one most seen by users.

So I propose the attached patch.