ifconfig: code shrink

adjtimex: code shrink
libbb: move nth_string function into libbb
hdparm: nth_string was here

   text    data     bss     dec     hex filename
 730013   10334   12032  752379   b7afb busybox_old
 730093   10134   12032  752259   b7a83 busybox_unstripped

libbb/compare_string_array.c

@@ -67,3 +67,12 @@ int index_in_substrings(const char *strings, const char *key)
 	}
 	return -1;
 }
+
+const char *nth_string(const char *strings, int n)
+{
+        while (n) {
+                n--;
+                strings += strlen(strings) + 1;
+        }
+        return strings;
+}