Add mg_next_query_string_entry_n() and mg_url_decode_n()

Move to mg_util.h so encode and decode are next ot each other.

Pull out mg_next_list_entry_n() for advanced use cases.

Add unit tests.

src/mg_util.h

@@ -205,6 +205,27 @@ struct mg_str mg_url_encode_opt(const struct mg_str src,
 /* Same as `mg_url_encode_opt(src, "._-$,;~()/", 0)`. */
 struct mg_str mg_url_encode(const struct mg_str src);
 
+/*
+ * Decodes a URL-encoded string.
+ *
+ * Source string is specified by (`src`, `src_len`), and destination is
+ * (`dst`, `dst_len`). If `is_form_url_encoded` is non-zero, then
+ * `+` character is decoded as a blank space character. This function
+ * guarantees to NUL-terminate the destination. If destination is too small,
+ * then the source string is partially decoded and `-1` is returned.
+ * Otherwise, the length of the decoded string is returned,
+ * not counting final NUL.
+ */
+int mg_url_decode(const char *src, int src_len, char *dst, int dst_len,
+                  int is_form_url_encoded);
+
+/*
+ * mg_str variant of mg_url_decode. Does not NUL-terminate dst.
+ * It is ok for src and dst to be the same.
+ */
+int mg_url_decode_n(struct mg_str src, struct mg_str *dst,
+                    int is_form_url_encoded);
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */