Refactor routines to stop/unstop processes and save the corresponding signal.

Refactor routines to stop/unstop processes and save the corresponding signal.

- Move the "hack" involving P_SINTR to avoid grabbing the SCHED_LOCK()
recursively closer to where it is necessary, in proc_stop()

- Introduce proc_unstop(), the symmetric routine to proc_stop(), which
manipulates `ps_xsig' and use it whenever a SSTOPed thread needs to be
awaken.

- Manipulate `ps_xsig' only in proc_stop/unstop()

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