kbfs "mountdir" config must not include leading slash

[mchelen-gov]

Setting a "mountdir" value that includes a leading slash causes an incorrect path directory.

For example:

keybase config set mountdir /media/myfiles/

results in a kbfs path with duplicate slashes such as /media/myfiles//private/username and cannot be opened in file explorer

docs reference: https://book.keybase.io/guides/linux#configuring-kbfs keybase version: 5.5.1-20200527202541+39ca0071e5 OS: Ubuntu 20.04.1 LTS