This adds a new way to enable safestack by listing compatible
ports in a file that is stored in hardenedbsd/ports_with_safestack.
This is an initial version, the final idea being to no longer need
to use 'USE_HARDENING' in the Makefiles of the many ports in order
to reduce our maintenance burden due to the merge conflict.
Signed-off-by: Loic <loic.f@hardenedbsd.org>