Configurable mount point/drive letter for kbfs in GUI.

Under linux i have tested and kbfsfuse allows me to mount the kbfs fuse filesystem in any arbitrary location, ~/keybase for example, yet from the GUI you cannot configure where to mount it (or tell it where it is mounted). I also use a windows PC which has K: already mapped to a network drive which hides kbfs on K:.

It would be nice if that location was configurable, even if only from the command line or a configuration file.

Start by reviewing the GUI mount configuration and the kbfsfuse behavior described in the issue, including the command-line or configuration-file options. Determine how mount locations are selected on Linux and drive letters on Windows. Done means users can configure the location and avoid conflicts such as an existing K: mapping.

Written by the indexing model from the issue text.