Semi-reversion of always-exit-from-sigthread change of 2005-09-15.

Semi-reversion of always-exit-from-sigthread change of 2005-09-15.
* exceptions.cc (sigpacket::process): Eliminate return after call to reinstated
noreturn function.
(signal_exit): Allow function to exit when a captive process has been
terminated.
* pinfo.cc (pinfo::exit): Enter exit_lock here.  Once again exit here under
control of exit_lock.
* sigproc.cc (sig_send): Don't wait for completion if process is exiting.
Remove special __SIGEXIT accommodations.
(wait_sig): Just exit the thread when a __SIGEXIT has been detected.  Don't
exit the process.