MFC kernel fixes to userland dtrace support.

r247049
-------
Avoid panic when tearing down the DTrace pid provider for a
process that has crashed.

sys/cddl/contrib/opensolaris/uts/common/dtrace/fasttrap.c:
	In fasttrap_pid_disable(), we cannot PHOLD the proc
	structure for a process that no longer exists, but
	we still have other, fasttrap specific, state that
	must be cleaned up for probes that existed in the
	dead process.  Instead of returning early if the
	process related to our probes isn't found,
	conditionalize the locking and carry on with a NULL
	proc pointer.  The rest of the fasttrap code already
	understands that a NULL proc is possible and does
	the right things in this case.

r247820
-------
Fix assertion failure when using userland DTrace probes from
the pid provider on a kernel compiled with INVARIANTS.

sys/cddl/contrib/opensolaris/uts/intel/dtrace/fasttrap_isa.c:
	In fasttrap_probe_pid(), attempts to write to the
	address space of the thread that fired the probe
	must be performed with the process of the thread
	held.  Use _PHOLD() to ensure this is the case.

	In fasttrap_probe_pid(), use proc_write_regs() instead
	of calling set_regs() directly.  proc_write_regs()
	performs invariant checks to verify the calling
	environment of set_regs().  PROC_LOCK()/UNLOCK() around
	the call to proc_write_regs() so that it's invariants
	are satisfied.

Sponsored by:	Spectra Logic Corporation
MFC r247048.

Orphaned processes that are being traced are killed by the
kernel.  Properly restore, continue, and detach from processes
being DTraced when DTrace exits with an error so the program
being inspected is not terminated.

cddl/contrib/opensolaris/cmd/dtrace/dtrace.c:
	In fatal(), the generic error handler, close the DTrace
	handle as is done in the "probe/script" error handler
	dfatal().  fatal() can be invoked after DTrace attaches
	to processes (e.g. a script specified by command line
	argument can't be found) and closing the handle will
	release them.

Submitted by:	Spectra Logic Corporation
Reviewed by:	rpaulo, gnn