stay in tstop() as long as the process is stopped

stay in tstop() as long as the process is stopped

Under some conditions (mainly related to multi-threaded processes and
tracing (gdb)), wakeups, scheduling and stops can lead to a race which
will leave the process stopped and wait()ed, but the P_WAITED flag
cleared.  This happens because a thread in tstop() might have been woken
up, but not yet scheduled.  If the process in turn would get stopped
again (another bug), the thread in tstop() would be counted as stopped,
but would only be waiting to be scheduled to transition into LSRUN.

Prevent this by checking for SSLEEP in tstop() before returning.