cp: use copy_file_range(2)

This has three advantages over write(2)/read(2):

• Fewer context switches and data copies
• Mostly preserves a file's sparseness
• On some file systems (currently NFS 4.2) the file system will perform the copy in an especially efficient way.

Reviewed by: rmacklem
MFC after: 2 weeks
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D26377