lavu: add locale-independent sscanf implementation

Copied and adopted from musl implementation.
 * converted all 'long double' to 'double'
 * removed %m support

libavutil/Makefile

@@ -94,6 +94,7 @@ OBJS = adler32.o                                                        \
        aes_ctr.o                                                        \
        audio_fifo.o                                                     \
        avstring.o                                                       \
+       avsscanf.o                                                       \
        base64.o                                                         \
        blowfish.o                                                       \
        bprint.o                                                         \

libavutil/avstring.h

@@ -400,6 +400,12 @@ int av_utf8_decode(int32_t *codep, const uint8_t **bufp, const uint8_t *buf_end,
  */
 int av_match_list(const char *name, const char *list, char separator);
 
+/**
+ * See libc sscanf manual for more information.
+ * Locale-independent sscanf implementation.
+ */
+int av_sscanf(const char *string, const char *format, ...);
+
 /**
  * @}
  */