change the hardcoded error constant (0x80000000UL) to a nice flexible define (BB_GETOPT_ERROR)

include/libbb.h

@@ -151,6 +151,7 @@ extern FILE *bb_xfopen(const char *path, const char *mode);
 extern int   bb_fclose_nonstdin(FILE *f);
 extern void  bb_fflush_stdout_and_exit(int retval) __attribute__ ((noreturn));
 
+#define BB_GETOPT_ERROR 0x80000000UL
 extern const char *bb_opt_complementaly;
 extern const struct option *bb_applet_long_options;
 extern unsigned long bb_getopt_ulflags(int argc, char **argv, const char *applet_opts, ...);