Fix a few small bugs in the DTrace USDT rules:
* anchor search strings appropriately,
* use .ALLSRC to pass the full path to the D script to dtrace(1),
* don't insert the auto-generated header into SRCS - it doesn't
  accomplish anything, and we end up having to remove it from OBJS anyway.

Reviewed by:		rpaulo
Differential Revision:	https://reviews.freebsd.org/D978
MFC after:		3 weeks
Sponsored by:		EMC / Isilon Storage Division