utf8.[ch] et al: use char32_t and char16_t instead of int, int32_t, int16_t

utf8.[ch] et al: use char32_t and char16_t instead of int, int32_t, int16_t

rework C11 utf8.[ch] to use char32_t instead of uint32_t when referring
to unicode chars, to make things more expressive.

[
 @zonque:
  * rebased to current master
  * use AC_CHECK_DECLS to detect availibility of char{16,32}_t
  * make utf8_encoded_to_unichar() return int
]