Replace all usage of strcasecmp/strncasecmp

Replace all usage of strcasecmp/strncasecmp

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: Martin Storsjö <martin@martin.st>