split unicodedomino_kernel_fixup_f4_and_checkseq.def:

• rename merged part (10FFFF upper bound in \parse@XML@charref
  and fix "F4 lead octet) ⇒ unicodedomino_kernel_fixup_f4.def
• move not merged part (better \UTFviii@checkseq) together with
  the remaining PR to unicodedomino_kernel_better_decode.def
• reorder inclusion; for now, include fixup_f4 unconditionally