tests/sendfile: test operation on unix/stream socket

Although there are already multiple tests in the tests collection
that utilize sendfile(2) support over unix/stream socket, they all
don't exercise the asynchronous part of the operation. This test
framework, however, uses a trick to toggle true async operation and
guarantee that pr_ready method of unix/stream is also tested.

Reviewed by: chs
Differential Revision: https://reviews.freebsd.org/D45055

tests/sendfile: factor out tcp_socketpair()

It creates a pair of connected TCP sockets for later testing. No
functional change.

Reviewed by: chs
Differential Revision: https://reviews.freebsd.org/D45054

Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

add a few tests for sendfile.

Reviewed by: markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D25431