Replace per thread P_CONTINUED with per process PS_CONTINUED flag

Replace per thread P_CONTINUED with per process PS_CONTINUED flag

dowait6() can only look at per process state so switch this over.
Right now SIGCONT handling in ptsignal is recursive and not quite
right but this is a step in the right direction. It fixes dowait6()
handling for multithreaded processes where the main thread exited.

OK mpi@