Changes:

	- fix UTRACE_SIGNAL_HOLD logic

	- don't assume QPassSignals: passes valid_signal()

	- implement qXfer:siginfo:read

	- implement continue with signal.

	  Note: it is still not clear to me what "signal SIG" should
	  do if the tracee did not report a signal (T00 and no signal
	  context). Currently, in this case ugdb just sends this sig
	  to the tracee, this means another TSIG report.

	  Another corner case. Consider

	  	(gdb) signal SIG &
	  	(gdb) interrupt

	  it is possible (not in practice, I think) that "interrupt"
	  will try to stop the tracee before it handles SIG, in this
	  case the tracee can report TSIG too.

Oleg.