salloc: clean up stopped child processes

This fixes a bug which is thanks to a report by Don Albert.

The problem is that whenever salloc exits with a child process in stopped state
(suspended or stopped on terminal input/output), a zombie process is generated,
since this case is not caught by the code evaluating the child status.

This patch adds the missing case.  It uses SIGKILL, which is the only signal
that changes the state of a stopped process. It was decided not to try and
re-awken the process using SIGCONT, since (a) this happens during session
clean-up and (b) if the condition is due to SIGTTIN, the process immediately
becomes stopped again.
Patch from Gerrit Renker, CSCS.