Replace all strcasecmp/strncasecmp usages.

All current usages of it are incompatible with localization.
For example strcasecmp("i", "I") != 0 is possible, but would
break many of the places where it is used.
Instead use our own implementations that always treat the data
as ASCII.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:   2011-04-18
 
 API changes, most recent first:
 
+2011-11-03 - xxxxxxx - lavu 51.23.0
+  Add av_strcasecmp() and av_strncasecmp() to avstring.h.
+
 2011-10-20 - b35e9e1 - lavu 51.22.0
   Add av_strtok() to avstring.h.