local: fix mkdir error when trying to copy files to the root of a drive on windows
local: fix mkdir error when trying to copy files to the root of a drive on windows

This was causing errors which looked like this when copying a file to
the root of a drive:

    mkdir \\?: The filename, directory name, or volume label syntax is incorrect.

This was caused by an incorrect path splitting routine which was
removing \ of the end of UNC paths when it shouldn't have been.  Fixed
by using the standard library `filepath.Dir` instead.

B4dM4n/rclone,ncw/rclone,ncw/rclone,B4dM4n/rclone,ncw/rclone,olihey/rclone,olihey/rclone,olihey/rclone,olihey/rclone,B4dM4n/rclone,B4dM4n/rclone,ncw/rclone