Fix mkdir_recursive on windows when CWD is root

Fix mkdir_recursive on windows when CWD is root

On windows, when creating a directory with an absolute path,
mkdir_recursive would start by trying to make 'C:'.

On windows, this has a special meaning, which is "the current
directory on the C drive". So the first thing it tries to do
is create the current directory.

Most of the time, this fails with EEXIST, so the function
continues as expected. However if the current directory is
C:/, trying to create that causes EPERM, which causes the
function to prematurely terminate.

(The same applies for any drive letter.)

This patch resolves this issue, by skipping the drive letter
so that it is never sent to the mkdir call.

Approved-By: Tom Tromey <tom@tromey.com>