Install BSD.tests.mtree when MK_TESTS is yes.

Problem found by jhb@.

Approved by:	rpaulo (mentor)
Move all atf directories to the tests mtree.

This is to ensure that test-related directories don't get needlessly
created (and later deleted) when MK_TESTS=no.

Problem found by jhb@.

Approved by:	rpaulo (mentor)
Use SCTP_PR_SCTP_TTL when the user provides a positive
timetolive in sctp_sendmsg().

MFC after: 3 days