Extend the internal JTAG event handlers to cover enable/disable,
and use those events to make sure that targets get "examined" if
they were disabled when the scan chain was first set up:

 - Remove "enum jtag_tap_event", merge with "enum jtag_event",
   so C code can now listen for TAP enable/disable events.

 - Report those events so they can trigger callbacks.

 - During startup, make target_examine() register a handler to
   catch ENABLE events for any then-disabled targets.

This fixes bugs like "can't halt target after enabling its TAP".

One class of unresolved bugs:  if the target has an ETM hooked
up to an ETB, nothing activates the ETB.  But starting up the
ETM without access to the ETB registers fails...


git-svn-id: svn://svn.berlios.de/openocd/trunk@2251 b42882b7-edfa-0310-969c-e2dbd0fdcd60