Add makesyscalls.lua, a rewrite of makesyscalls.sh

This currently requires a suitable lua + luafilesystem + luaposix from the
ports tree to build. Discussion is underway in D21893 to add a suitable lua
to the base system, cleverly disguised and out of the way of normal
consumers.

makesyscalls.sh is a good target for rewrite into lua as it's currently a
sh+sed+awk script that can be difficult to add on to, at times. For
instance, adding a new COMPAT* option (that mimicks the behaivor of most
other COMPAT* options) requires a fairly substantial amount of copy/paste;
see r352693 for instance. Attempts to generate part of the awk script for
COMPAT* handling was (very kindly) rejected with a desire to just rewrite
the script in a single language that can handle all of it.

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D21775