avutil/avstring: Factor av_match_list() out

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/avstring.h

@@ -357,6 +357,13 @@ int av_escape(char **dst, const char *src, const char *special_chars,
 int av_utf8_decode(int32_t *codep, const uint8_t **bufp, const uint8_t *buf_end,
                    unsigned int flags);
 
+/**
+ * Check if a name is in a list.
+ * @returns 0 if not found, or the 1 based index where it has been found in the
+ *            list.
+ */
+int av_match_list(const char *name, const char *list, char separator);
+
 /**
  * @}
  */