drive: win: add hotplug support.

__Command line argument:__ _/drive:*,hotplug_
Actually, "_hotplug_" in the command line argument is a fake path, and can be any word. Only "\*" is in service.
This argument provides the same function as "Drives that I plug in later" option in MSTSC.
__Known issue:__
  If you have a usb drive, whose drive name is "F", redirected to RD, using "_/drive:f,f+/ /drive:*,hotplug_", it will works well when you plug-out the usb drive. But if you use another name for redirecting the usb drive, taking "_/drive:aname,f+/ /drive:*,hotplug_" as an example, the drive symble will not be deleted when you plug-out the usb drive.