posix-style device names in mount

[Kazuhiro Fujieda]

mount.exe can pass a posix style path as the device name to the
mount system call. But It isn't supported by the system call. It
rarely cause problems because the device name is simply
backslashified in mount_info::add_item().

But in the following case on the latest snapshot...

$ mount //d/text /text
$ mount
\\d\tex             /text               user         textmode
C:                  /                   user         binmode
$ ls /text
ls: /text: No such device or address

If mount.exe or the mount system call converted the device name
to the win32 style path, this problem wouldn't occur.
____
  | AIST      Kazuhiro Fujieda <fujieda@jaist.ac.jp>
  | HOKURIKU  School of Information Science
o_/ 1990      Japan Advanced Institute of Science and Technology