Automatically build and link DTrace USDT probes.

This completely replaces bsd.dtrace.mk which never worked reliably.

MFC after:	3 weeks
Remove _DTRACE_VERSION from sdt.h.  It will now come from the command line
(bsd.dep.mk).

MFC after:	3 weeks
Move iic.h to sys/ so that it's automatically installed in /usr/include/sys.

This lets us call iic(4) ioctls without needing the kernel source code
and follows the same model of GPIO.

MFC after:	3 weeks
Optimisation: replace ${_D:R} with just ${_D}.

MFC after:	3 weeks