v68k:  Set CCR in enter_supervisor_mode_callback()

Set N if the operation was refused, otherwise clear N.
Set Z if the new SR equals the old SR, otherwise clear Z.
vdb:  Check enter_supervisor_mode() result

Don't try to access the privileged Trace exception vector address if the
enter_supervisor_mode() call failed and we're still in user mode.
vdb:  Return 0 or -1 from set_trace_handler()
vdb:  Bail if we couldn't enter supervisor mode
vdb:  Return 1, not 0, if supervisor mode declined
Merge branch 'vdb-supervisor-mode-check'