of_asprintf: Change %k to %C and %K to %S.

This reverts 2943b43. It turned out that using %C and %S is possible,
even when mixing code with Cocoa code, as it is possible to introduce
__attribute__((format(__OFString__, ...))) to Clang which just handles
%C and %S as being of_unichar_t instead of unichar like they are handled
for __attribute__((format(__NSString__, ...))).

The Clang patch for __attribute__((format(__OFString__, ...))) has been
submitted, but is not upstream yet, thus the changes for that are not
being committed yet.