revert last two commits. vfork cannot be used in subroutine,

it trashes stack on return

include/libbb.h

@@ -719,11 +719,6 @@ int bb_execvp(const char *file, char *const argv[]) FAST_FUNC;
 #define BB_EXECLP(prog,cmd,...) execlp(prog,cmd, __VA_ARGS__)
 #endif
 
-#if BB_MMU
-pid_t xfork(void) FAST_FUNC;
-#endif
-pid_t xvfork(void) FAST_FUNC;
-
 /* NOMMU friendy fork+exec */
 pid_t spawn(char **argv) FAST_FUNC;
 pid_t xspawn(char **argv) FAST_FUNC;