pksubprocess signal race condition

[robnagler]

The signal handler should collect the signals and waitpid should deliver them. The signal may occur before subprocess.Popen returns, that is, the child (from pksubprocess_test) executes before the parent process has gotten the pid from subprocess. This occurs in parallel test runs. Simplifying the signal handler is better anyway. This will require a change to the way the non-recursive waitpid works. It's ok to poll. The key is making it fast enough that higher level processes (e.g. sirepo job supervisor) do not send their secondary signal before the signal gets to be cascaded. This should be fine. The poll loop can actually be structured so that the check for signals happens every .1 and the ps calls happen every 1 second.