Rationalize Query_for_list_of_[relations] query names in tab-complete.c.

The previous convention was to use names based on the set of relkinds being
selected for, which was not at all helpful for maintenance, especially
since people had been quite inconsistent about whether to change the names
when they changed the relkinds being selected for.  Instead, use names
based on the functionality we need the relation to have, following the
model established by Query_for_list_of_updatables.

While at it, sort the list of Query constants a bit better; it had the
distinct air of code-assembled-by-dartboard before.

Discussion: https://postgr.es/m/14830.1537481254@sss.pgh.pa.us