This replaces the "TailMatchesN" macros with just "TailMatches",
and likewise "HeadMatchesN" becomes "HeadMatches" and "MatchesN"
becomes "Matches".  The various COMPLETE_WITH_LISTn macros are
reduced to COMPLETE_WITH, and the single-item COMPLETE_WITH_CONST
also gets folded into that.  This eliminates a lot of minor
annoyance in writing tab-completion rules.  Usefully, the compiled
code also gets a bit smaller (10% or so, on my machine).

The implementation depends on variadic macros, so we couldn't have
done this before we required C99.

Andres Freund and Thomas Munro; some cosmetic cleanup by me.

Discussion: https://postgr.es/m/d8jo9djvm7h.fsf@dalvik.ping.uio.no