split: new -t option to select record separator

split: new -t option to select record separator

* src/split.c (eolchar): A new variable to hold
the separator character (unibyte for now).
This is reference throughout rather than hardcoding '\n'.
(usage): Describe the new --separator option, and
mention records along with lines so there is no ambiguity
that all options treat lines and records equivalently.
(main): Have -t update eolchar, or default to '\n'.
* tests/split/record-sep.sh: New test case.
* tests/local.mk: Reference the new test.
* doc/coreutils.texi (split invocation): Document the new option.
Adjust --lines, --line-bytes, --number=[lr]/... to mention
they pertain to records if --separator is specified.
* NEWS: Mention the new feature.