fix pantsd deadlock with ctrl-c while awaiting pid

[cosmicexplorer]

It's pretty easy to press Ctrl-C while the RemotePantsRunner client is waiting for the pantsd pid, within a time window which causes an immediate deadlock, requiring killing pantsd in another terminal.

I went over this with @blorente yesterday in the context of #6574, but I now believe it will probably have to be addressed as part of #6579.

[cosmicexplorer]

I believe this may be a duplicate of #5220, except that that ticket describes pants running to completion, while this ticket describes a deadlock in which the terminal is frozen and pants also refuses to run. I suspect these are closely related events, and I intend to fix both in #6579.

[blorente]

I think this is fixed in #7924