hProcess handle here since it is used to ensure that a new process isn't
created with the old pid after the old pid exits.
* spawn.cc (spawn_guts): Ditto.
* pinfo.cc (proc_waiter): Don't send any signals if we've execed since this
process doesn't officially exist.
* pinfo.h (pinfo::pid_handle): Eliminate. Just use hProc.
* sigproc.cc (sig_send): Don't send any signals if our sendsig doesn't exist.
That's a sign that we are execing.
(remove_proc): Eliminate pid_handle close.
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