avutil/avstring: add av_strreplace API into avstring

refer to: http://creativeandcritical.net/str-replace-c
add av_strreplace API for replace string operations.

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavutil/avstring.h

@@ -266,6 +266,11 @@ int av_strcasecmp(const char *a, const char *b);
  */
 int av_strncasecmp(const char *a, const char *b, size_t n);
 
+/**
+ * Locale-independent strings replace.
+ * @note This means only ASCII-range characters are replace
+ */
+char *av_strreplace(const char *str, const char *from, const char *to);
 
 /**
  * Thread safe basename.