unifdef(1): Improve worst-case bound on symbol resolution
Use RB_TREE to make some algorithms O(lg N) and O(N lg N) instead of O(N)
and O(N^2).
While here, remove arbitrarily limit on number of macros understood.
Reverts r354877 and r354878, which disabled the (correct) test.
PR: 242095
Reported by: lwhsu
minus the vendor Makefiles
Provide directions for how to bootstrap the vendor sources in
FREEBSD-upgrade
MFC after 2 weeks
Discussed with: rpaulo
Sponsored by: EMC / Isilon Storage Division
