truncate: support sizes relative to an existing file

truncate: support sizes relative to an existing file

* doc/coreutils.texi (truncate invocation): Mention that --reference
bases the --size rather than just setting it.
* src/truncate.c (usage): Likewise. Also remove the clause
describing --size and --reference as being mutually exclusive.
(do_truncate): Add an extra parameter to hold the size
of a referenced file, and use it if positive.
(main): Pass the size of a referenced file to do_truncate().
* tests/misc/truncate-parameters: Adjust for the new combinations.
* NEWS: Mention the change
Suggested by Richard W.M. Jones