Further work on tab completion code: arrange for system catalogs to be

Further work on tab completion code: arrange for system catalogs to be
offered for completion only when the input-so-far is at least 'pg_'.
This seems to be the best compromise behavior emerging from yesterday's
discussion.  While at it, refactor code to eliminate repetitive use of
nearly identical queries, which was exceedingly tedious to maintain.
Also const-ify code more thoroughly in hopes of moving constant data into
text segment, and remove unnecessary length limit on queries.