libsubprocess: handle SIGCONT

[chu11]

libsubprocess handles SIGSTOP and can return a STOPPED state to the user, but it doesn't return RUNNING if a SIGCONT is sent. I would think this should be handled to balance it out? Or should it be a new state that's not RUNNING?

Note, this doesn't seem that important but seems more of "well, you should probably handle this too" b/c of #5083

[grondo]

Maybe CONTINUED to match waitpid(2)? Not sure if there's a use case for this though..