Split bridge functions into a new module.

This patch is just:
   * Code movement
   * Adding headers here and there as needed
   * Adding a bridges_free_all() with a call to it.

It breaks compilation, since the bridge code needed to make exactly
2 calls into entrynodes.c internals.  I'll fix those in the next
commit.

src/test/test_controller.c

@@ -3,6 +3,7 @@
 
 #define CONTROL_PRIVATE
 #include "or.h"
+#include "bridges.h"
 #include "control.h"
 #include "entrynodes.h"
 #include "networkstatus.h"