Allow unquoted UTF-8 HERE-document terminators

Allow unquoted UTF-8 HERE-document terminators

When not explicitly quoted, tokenization of the HERE-document terminator
dealt improperly with multi-byte characters, advancing one byte at a
time instead of one character at a time.  This lead to
incomprehensible-to-the-user errors of the form:

    Passing malformed UTF-8 to "XPosixWord" is deprecated
    Malformed UTF-8 character (unexpected continuation byte 0xa7, with
      no preceding start byte)
    Can't find string terminator "EnFra�" anywhere before EOF

If enclosed in single or double quotes, parsing was correctly effected,
as delimcpy advances byte-by-byte, but looks only for the single-byte
ending character.

When doing a \w+ match looking for the end of the word, advance
character-by-character instead of byte-by-byte, ensuring that the size
does not extend past the available size in PL_tokenbuf.