makesyscalls.lua: Add a new syscall type: RESERVED

RESERVED syscall number are reserved for local/vendor use. RESERVED is
identical to UNIMPL except that comments are ignored.

Reviewed by: kevans
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27988

(cherry picked from commit 119fa6ee8a8056aab5e4ab1719d3c563cdb4a95a)