KBFS on macOS Catalina throws errors about missing /keybase

macOS Catalina, with its brand new read-only root volume, doesn't allow for mounting arbitrary volumes at the root. This means when Keybase tries to mount the /keybase volume it ends up mounting at /Volumes/Keybase instead, and then the Keybase GUI throws errors about not finding /keybase.

This can be solved by adding a line consisting of the word keybase to /etc/synthetic.conf and then restarting the machine. That enables the ability to mount volumes at /keybase, though the requirement of a restart is unfortunate.

At the very least, Keybase should learn to detect this mounting issue on Catalina, and support the mount at /Volumes/Keybase if it can't mount at /keybase. It should ideally prompt the user if they want to enable mounting at /keybase, with a warning that it requires a reboot, and if the user agrees then prompt if they want to reboot now or later. But merely detecting the mount failure and accepting /Volumes/Keybase should be the first step.

Start by reproducing the mount behavior on macOS Catalina and trace the Keybase mount handling and GUI path checks mentioned in the issue. Done means the client detects the /keybase failure, accepts /Volumes/Keybase, and avoids the missing-mount error; prompting for synthetic.conf and rebooting is a later enhancement.