Update WITH_/WITHOUT_SSP descriptions

ProPolice refers to a specific implementation by Hiroaki Etoh and
Kunikazu Yoda. The implementation in contemporary Clang and GCC is
somewhat different and newer, so use a generic term in the src.conf
descriptions.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit b461276d75488fe50c1503db85d43cbb8c3c3067)