Convert `u_char` instances to `unsigned char` to improve portability

`u_char` is not in the C standard, and so this fails to build on OSX.
It could be fixed by including `sys/types.h`, but it seems easier just
to write it out as `unsigned char` instead of pulling in yet another
header file.
Merge branch 'u_char_removal' of https://github.com/avsm/libmacaroons

* 'u_char_removal' of https://github.com/avsm/libmacaroons:
  Convert `u_char` instances to `unsigned char` to improve portability