ktls_test: Permit an option to skip tests not using ifnet TLS.

If ktls.require_ifnet is set to true, then check the TLS offload mode
for tests sending and receiving records and skip the test if the
offload mode is not ifnet mode.

This can be used along with ktls.host to run KTLS tests against a NIC
supporting ifnet TLS and verify that expected cipher suites and
directions used ifnet TLS rather than software TLS. Receive tests may
result in a false positive as receive ifnet TLS can use software as a
fallback.

Reviewed by: markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D35427