cp: add --reflink=never to force standard copy mode

cp: add --reflink=never to force standard copy mode

This mode is currently the default, but most if not all users of
reflink-capable filesystems want --reflink=auto, which is often
encapsulated into an alias.  Adding --reflink=never allows overriding
such an alias.

* doc/coreutils.texi (cp invocation): Describe the new option.
* src/cp.c: Support --reflink=never.
* tests/cp/reflink-auto.sh: Add a test case.
* NEWS: Mention the new feature.