Convert tab-complete's long else-if chain to a switch statement.

Rename tab-complete.c to tab-complete.in.c, create the preprocessor
script gen_tabcomplete.pl, and install Makefile/meson.build rules
to create tab-complete.c from tab-complete.in.c.  The preprocessor
converts match_previous_words' else-if chain into a switch and
populates tcpatterns[] with the data needed by the driver loop.

The initial HeadMatches/TailMatches/Matches test in each else-if arm
is now performed in a table-driven loop.  Where we get a match, the
corresponding switch case is invoked to see if the match succeeds.
(It might not, if there were additional conditions in the original
else-if test.)

The total number of string comparisons done is just about the
same as it was in the previous coding; however, now that we
have table-driven logic underlying the handmade rules, there
is room to improve that.  For now I haven't bothered because
tab completion is still plenty fast enough for human use.
If the number of rules keeps increasing, we might someday
need to do more in that area.

The immediate benefit of all this thrashing is that C compilers
frequently don't deal well with long else-if chains.  On gcc 8.5.0,
this reduces the compile time of tab-complete.c by about a factor of
four, while MSVC is reported to crash outright with the previous
coding.

Discussion: https://postgr.es/m/2208466.1720729502@sss.pgh.pa.us

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