session: Use KBFS_MOUNT (#81)
session: Use KBFS_MOUNT (#81)

Don't rely on a config setting; KBFS's mountpoints
on macOS and Linux have diverged sufficiently as to make
doing so unreliable.
woodruffw/kbsecret,kbsecret/kbsecret,kbsecret/kbsecret