bsd-user/signal.c: implement force_sig_fault

bsd-user/signal.c: implement force_sig_fault

Start to implement the force_sig_fault code. This currently just calls
queue_signal(). The bsd-user fork version of that will handle this the
synchronous nature of this call. Add signal-common.h to hold signal
helper functions like force_sig_fault.

Signed-off-by: Stacey Son <[email protected]>
Signed-off-by: Kyle Evans <[email protected]>
Signed-off-by: Warner Losh <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>