Cygwin, support Windows-style parameter paths

[benrubson]

Hi,

This PR enables the use of Windows-style paths as parameters.
Now, user can use for example :
encfs C:\cipher X:

Thank you 👍

Ben

[benrubson]

So as you can see here I had to create a new internal configuration item, unmountPoint, to retain mount point as originally provided by the user.
In Cygwin, pkill is used to unmount, and in order to identify the correct encfs process to stop (one could mount several FS at the same time), pkill looks at the whole encfs command line for the given mount point.
Unfortunately, the existing internal configuration item mountPoint is updated (from Win to Linux if needed, and with a trailing slash), so can't be used to unmount, as it would have chances not to match existing processes.

I did not found an easier/shorter way to solve this issue.