mtree entries for the DTrace tests.
Commit what I have so far.
Don't define targets if PROGS happens to be empty.
Fix some bugs in the dtrace rules from bsd.dep.mk:

- Don't include the generated header in SRCS.
- Be more strict with pattern matching.
- Use .ALLSRC to ensure that we can find objects in the make path.
Merge branch '20141012-dtrace-kyua2' of github.com:markjdb/freebsd into 20141012-dtrace-kyua2

Conflicts:
	cddl/usr.sbin/dtrace/Makefile
	cddl/usr.sbin/dtrace/tests/Makefile
	etc/mtree/BSD.tests.dist