Clear ps_xsig when continuing after a PS_TRACED stop.

Clear ps_xsig when continuing after a PS_TRACED stop.

Also remove the ps_xsig handling in setrunnable() it is in the wrong spot
and causes signals to be delivered over and over again.

Attaching to an already stopped process is affected by this. The SIGSTOP
sent by ptrace is now ignored in ptsignal() and as a result gdb will hang
in wait4() until a SIGCONT is delivered to the process. After that all
works as usual.

OK mpi@