Move signal stuff into src/lib/empthread/, no functional change:

(empth_init_signals): New, factored out of start_server().  Call from
empth_init().
(panic): Move to posix.c, internal linkage.
(empth_obj): Add new posix.o

(empth_start, empth_alarm) [EMPTH_POSIX]: Clean up pointless messing
with signal handlers.
(empth_init, empth_wakeup) [EMPTH_POSIX]: Clean up a bit.

src/lib/empthread/lwp.c

@@ -47,6 +47,7 @@ int
 empth_init(void **ctx, int flags)
 {
     empth_flags = flags;
+    empth_init_signals();
     empth_main = lwpInitSystem(PP_MAIN, (char **)ctx, flags);
     return 0;
 }