Move messages.c to libbb. Make each string in messages.c be its own .o file.

This way, we can new get rid of all that tedious #define rubbish we used to
need to enable specific messages.  This way is enormously simpler, and as a
bonus also ends up saving us 96 bytes.
 -Erik

libbb/xfuncs.c

@@ -31,11 +31,6 @@
 #include <unistd.h>
 #include "libbb.h"
 
-/* same conditions as recursive_action */
-#define bb_need_memory_exhausted
-#define BB_DECLARE_EXTERN
-#include "../messages.c"
-
 
 #ifndef DMALLOC
 extern void *xmalloc(size_t size)