Tighten error handling on TAP enable/disable paths a bit:

 - Don't enable/disable unless it's necessary.  Those event
   handlers could have nasty side effects...

 - Don't *succeed* enables/disables if there was no code which
   could have implemented that action.  This prevents bugs like
   wrongly acting as if the scan chain changed.

 - Minor whitespace cleanup in enable/disable command code.

The big problem is still the lack of code to verify scan chains
were actually updated as requested; this adds a comment on that.
I suspect the best we can do near term will be to verify IDCODE.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2250 b42882b7-edfa-0310-969c-e2dbd0fdcd60