dtrace: cleanup (and adding) of SDT probe points

dtrace: cleanup (and adding) of SDT probe points

Changed io SDT probe points to be located at the buffer_head level rather than
the bio level.  This may need to be revisted depending on further analysis,
but doing it this way provides consistent semantics that were not guaranteed by
the previous bio-based placement.

Changed the sched STD probes to not pass irrelevant arguments, and to pass
specific runqueue CPU information.  The CPU information is not available from
the task structure, so it needs to be passed explicily.

Added proc SDT probes start and lwp-start.

Added proc SDT probes for signal-discard and signal-clear.

Corrected the argument to the exit proc SDT probe, which should indicate the
reason for the process termination (exit, killed, core dumped) rather than the
return code of the process.

Provided argument information for all the new (and changed) SDT probe points.
This depends on working xlator support in userspace.

Enabling of SDT probes now uses a generic dtrace_invop_(enable|disable) rather
than SDT-specific functions.

SDT probes are not destroyed correctly, to ensure that subsequent uses will not
result in unpleasant events.

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>