comm: support NUL --output-delimiter for consistency

comm: support NUL --output-delimiter for consistency

* src/comm.c (main): Track the output delimiter length,
so that it can be adjusted to 1 for the NUL delimiter.
Also rename the global variable from "delimiter" to
"col_sep" so its use is more obvious, and to distinguish
from the recently added "delim" global variable.
* tests/misc/comm.pl: Adjust accordingly.