ipfilter 5.1.2 no longer supports sysctl. Use ipf -V to determine if
available (the kernel module is loaded or compiled into the kernel).

Approved by:	glebius (mentor)
Approved by:	re (blanket)
Add a sendfile regression test for transmit length > file size.

This test identified the issue fixed in FreeBSD-SA-13:11.sendfile.

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