stest: If one of our secnets dies, call the test a failure

stest: If one of our secnets dies, call the test a failure

This involves use of TclX's `signal' facility.  In my tests it was
easy to make Tcl deadlock by doing too much work in the signal
handler.  In particular reaping children is a bad idea.  Also signals
are not blocked during the signal handler so it would have to be
reentrant.

Instead, use `after idle'.  That is quite soon enough for the reap to
run, and in my tests with TclX 8.4 it all works properly.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>