Also have special error handling for when branches are invalid and
ENOENT would be returned for getattr and readdir so users understand
what is going on and the runtime interface can still be used to fix
the problem.
If available FICLONE and copy_file_range will be tried in addition to sendfile
when copying data between two files. The fallback is a tradition read/write
loop. On systems that support these it should improve performance.
