cp: accept the --reflink option

cp: accept the --reflink option

* NEWS: Mention it.
* doc/coreutils.texi (cp invocation): Describe it.
* src/copy.h (struct cp_options) [reflink]: New member.
* src/copy.c (usage): Describe it.
(copy_reg): If reflink is true try to clone the file.
(main): Check for --reflink.
(cp_option_init): Initialize the new member.
* src/install.c (cp_option_init): Initialize the new member.
* src/mv.c (cp_option_init): Likewise.
* tests/cp/sparse: Add a new test case.