(empth_init_signals): Don't catch SIGINT and SIGTERM.

(empth_init_signals): Don't catch SIGINT and SIGTERM.
(empth_wait_for_shutdown): New.
(main): Use it to wait for shutdown signal, then shut down.  Closes
#770492.
(empth_exit): Remove the weird special case for main thread.

Implement empth_wait_for_shutdown() for EMPTH_LWP:
[EMPTH_LWP] (lwpInitSigWait, lwpSigWait, lwpSigWakeup): New.
Declaration of lwpSigWait was accidentally committed in the previous
revision of lwp.h.
[EMPTH_LWP] (lwpInitSystem): New parameter waitset, pass it on to
lwpInitSigWait().
[EMPTH_LWP] (lwpReschedule): Call lwpSigWakeup().
[EMPTH_LWP] (empth_init): Declare signals needed by
empth_wait_for_shutdown().
(empth_wait_for_shutdown): Implement on top of lwpSigWait().

Implement empth_wait_for_shutdown() for EMPTH_POSIX:
[EMPTH_POSIX] (empth_init): Block signals, so that
empth_wait_for_shutdown() can use sigwait() safely.
(empth_wait_for_shutdown): Implement on top of sigwait().

Implement empth_wait_for_shutdown() for EMPTH_W32:
(empth_wait_for_shutdown): Implement on top of loc_BlockMainThread().