Default KBFS mountpoint no longer created on Gentoo.

Using Keybase on Gentoo, noticed after an update that my KBFS folder in `/var/run/user/1000/keybase` no longer existed. Seems to be due to a change in the `run_keybase` script: https://github.com/keybase/client/commit/3d9214879ec4ba99f3dd6f136f150eb3edfbaaf8#diff-47650c75f7c44ae86a5b4806e122e18eL290. `$mountdir` creation was removed without a new version in the added lines of that patch. Problem's easy enough to fix by changing the mountdir or creating it and restarting the service, but it seems that the removal of that line may have been mistaken? Don't see anything about it in the commit message in any case.

Start with the run_keybase script and compare the $mountdir handling in commit 3d9214879ec4ba99f3dd6f136f150eb3edfbaaf8. Reproduce the service setup on Gentoo, then verify that /var/run/user/1000/keybase exists after restarting the service.

Written by the indexing model from the issue text.