Replace index_in_[sub]str_array with index_in_[sub]strings,

which scans thru "abc\0def\0123\0\0" type strings. Saves 250 bytes.

   text    data     bss     dec     hex filename
 781266    1328   11844  794438   c1f46 busybox_old
 781010    1328   11844  794182   c1e46 busybox_unstripped

networking/ipcalc.c

@@ -72,7 +72,7 @@ int get_prefix(unsigned long netmask);
 		"hostname\0"  No_argument "h"
 		"silent\0"    No_argument "s"
 # endif
-		"\0";
+		;
 #endif
 
 int ipcalc_main(int argc, char **argv);