OpenSSL: Only enable KTLS if it is explicitly configured

It has always been the case that KTLS is not compiled by default. However
if it is compiled then it was automatically used unless specifically
configured not to. This is problematic because it avoids any crypto
implementations from providers. A user who configures all crypto to use
the FIPS provider may unexpectedly find that TLS related crypto is actually
being performed outside of the FIPS boundary.

Instead we change KTLS so that it is disabled by default.

We also swap to using a single "option" (i.e. SSL_OP_ENABLE_KTLS) rather
than two separate "modes", (i.e. SSL_MODE_NO_KTLS_RX and
SSL_MODE_NO_KTLS_TX).

Reviewed by: jkim
Obtained from: OpenSSL (a3a54179b6754fbed6d88e434baac710a83aaf80)
MFC after: 5 days
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31440