Add the DTrace probe definitions for plockstat support.

This will be connected to the system later.

Sponsored by:	The FreeBSD Foundation
Correct format string to fix build of uhsoctl when DEBUG is defined

PR:		185007
Submitted by:	saper saper.info
MFC after:	3 days
Use %zu not %zd for printing size_t.

PR:		185007
Submitted by:	saper saper.info
MFC after:	3 days
bsd.dep.mk: add support for generating DTrace header files from .d files.

MFC after:	3 weeks
Check the number of returned entries before doing any access to ifm_ulist.