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.

include/empthread.h

@@ -211,4 +211,11 @@ void empth_sem_signal(empth_sem_t *sem);
  */
 void empth_sem_wait(empth_sem_t *sem);
 
+
+/*
+ * Stuff for implementations, not for clients.
+ */
+
+void empth_init_signals(void);
+
 #endif