Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Allow to specify path to a file we want to test with sendfile(2).
This allows to specify selected file system and not only /tmp/.

Update the sendfile regression test so that it outputs results in the
TAP format.
Add a checksum to verify that the data hasn't been corrupted between
being read from disk and being received.

Approved by: rrs (mentor)

o Correct a comment: a test file size is a four pages not three.

Test sending 0 bytes.

Fix length calculation.

Valid error codes for err() and errx() are 1..255.
The correct format specifier for ssize_t is %zd.

Add more tests to verify last sendfile(2) breakage: test sending more
than a page size and nbytes=0.

Add a basic regression test for sendfile() over TCP, which sends varying
lengths of headers and data and makes sure it receives about the right
number of bytes.