cp(1) could use sendfile

Review Request #1311 — Created Dec. 2, 2018 and updated — Latest diff uploaded


Change to libcmdutils so that cp(1) uses the sendfile(3ext) interface.
This avoids copying the data between kernel and userspace.

My testing so far includes using this cp on trees with various file
sizes. I've tested that checksums still match. I've tested error handling
is unchanged when reaching the fs quota. I also booted into a system with this change.
I have not tested this with NFS because I currently don't have such a setup.