bug#24400: cp: default to --reflink=auto?

[Paul Eggert]

Currently 'cp' defaults to not using reflinks, which can be 
significantly slower on a system that supports file cloning. I expect 
most users would prefer 'cp' to default to --reflink=auto for spped. 
Although there are some special cases otherwise, on balance how about if 
we change cp to default to --reflink=auto, and add an option 
--reflink=no for users who prefer the current default?