Use stdint.h to simplify type logic

See http://mailman.mit.edu/pipermail/krbdev/2014-February/011886.html

This branch does not go as far as globally substituting away the public krb5 fixed-width types (like krb5_int32 and krb5_ui_4) within our tree, as that would be annoying to revert if we find a problem.  But it does go as far as including <stdint.h> from <krb5.h> and using the C99 type names to define the krb5 fixed-width types.