dtrace: USDT implementation (phase 2)

dtrace: USDT implementation (phase 2)

This commit contains the 2nd phase of the USDT implementation for DTrace for
Linux.  It provides the mechanics of fasttrap probes (enabling, firing, and
disabling).  It also contains various debugging statements that will be cleaned
up in a future commit.  They exist right now, because this commit represents a
work in progress that is known to contain various bugs and loose ends.  The
commit for this code is provided as a "sharing of the pain" , and to ensure
that others can start playing with this code and test the interaction with
userspace.

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