openssl: install pc files

most programs in ports are looking for .pc files in order to get the
necessary information on how to compile and link against openssl.

The ports now also has a way to hide or force a path for pkgconf.

Providing .pc files along with openssl in base will allow (once all
the supported version of FreeBSD has it) so improve the framework to
deal with openssl in base vs openssl in ports (and libressl)

This will also greatly reduce the number of patches necessary to
workaround the build systems which only knows how to detect where
openssl is installed via pkgconf.

PR: 266051
MFC After: 3 weeks
Reviewed by: jkim, delphij
Exp-run by: antoine
Differential Revision: https://reviews.freebsd.org/D36360

(cherry picked from commit b323fa85f15268ac44b8ff90faf90bce5d87b608)