Decoupling loops from threads to allow fixed thread count

On large configurations, too many threads would spawn with one thread
per loop. Therefore, threads may now run multiple loops at once. The
thread count is configurable and may be changed during run. All threads
are spawned on startup.

This change helps with memory bloating. BIRD filters need large
temporary memory blocks to store their stack and also memory management
keeps its hot page storage per-thread.

Known bugs:
* Thread autobalancing is not yet implemented.
* Low latency loops are executed together with standard loops.

sysdep/unix/unix.h

@@ -16,6 +16,7 @@ struct pool;
 struct iface;
 struct birdsock;
 struct rfile;
+struct config;
 
 /* main.c */
 
@@ -32,6 +33,8 @@ void cmd_reconfig_undo(void);
 void cmd_reconfig_status(void);
 void cmd_shutdown(void);
 void cmd_graceful_restart(void);
+void cmd_show_threads(int);
+void bird_thread_commit(struct config *new, struct config *old);
 
 #define UNIX_DEFAULT_CONFIGURE_TIMEOUT	300