In addition to 2-byte and 3-byte UTF-8 sequences, correctly identify all

In addition to 2-byte and 3-byte UTF-8 sequences, correctly identify all
4-byte UTF-8 sequences and not just some of them, to keep them together
and avoid passing them on byte by byte, helping tools like tmux(1).

While here, also do all the range tests with < and > rather than &
for uniformity and readability, and add some comments.

Input and OK jca@ and nicm@.

Soeren at Soeren dash Tempel dot net originally reported the bug
and provided an incomplete patch that was used as a starting point,
and he also tested this final patch.