Move wchar.c and encnames.c to src/common/.

Move wchar.c and encnames.c to src/common/.

Formerly, various frontend directories symlinked these two sources
and then built them locally.  That's an ancient, ugly hack, and
we now have a much better way: put them into libpgcommon.
So do that.  (The immediate motivation for this is the prospect
of having to introduce still more symlinking if we don't.)

This commit moves these two files absolutely verbatim, for ease of
reviewing the git history.  There's some follow-on work to be done
that will modify them a bit.

Robert Haas, Tom Lane

Discussion: https://postgr.es/m/CA+TgmoYO8oq-iy8E02rD8eX25T-9SmyxKWqqks5OMHxKvGXpXQ@mail.gmail.com