Hardcoded Unix UID in config file

After migrating home user data to a new installation, I saw that kbfs won't start because there are hardcoded paths in the config file (Linux) which refer to the numeric UID of the user. It is best practice that programs do not rely on that UID being static (see migration issue). Instead, the paths should be determined at run-time.

Specifically, in `$HOME/.config/keybase/config.json`, `"mountdir": "/run/user/1000/keybase/kbfs"` should be something like `"mountdir": "/run/user/$UID/keybase/kbfs"`.

Keep on creating great software!

Start by tracing how the Linux client writes `$HOME/.config/keybase/config.json` and derives the `mountdir` value. Reproduce the migrated-user case described in the issue and verify that the resulting path is resolved for the current user rather than containing a hardcoded numeric UID.

Written by the indexing model from the issue text.