Log Message:
Retire 'djbware-errno-hack' and associated options.mk cleverness.
Instead:

1. Package makefiles including their own options.mk
2. Packages say "SUBST_CLASSES+=djberrno" to get the hack, if needed
3. Packages adjust SUBST_FILES.djberrno, if needed

Should fix bulk build failures due to multiple inclusions of options.mk
and/or incorrect definitions of DJB_ERRNO_HACK.

Approved during the freeze by wiz@.