paste: support multi-byte delimiters

paste: support multi-byte delimiters

* src/paste.c (collapse_escapes): This is the central --delimiters
parsing function, so adjust to handle multi-byte chars with
mcel_scanz().  Populate a delimiters length array to support
characters of differing lengths.
(paste_serial): Use the delimiters length array to output
the appropriate delimiter.
(paste_parallel): Likewise.
* tests/paste/multi-byte.sh: A new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the improvement.