vstart_runner: allow the use of it with kernel mounts

vstart_runner: allow the use of it with kernel mounts

Add a new command-line switch to allow it to use the kernel client
instead, and add all of the machinery to handle local kcephfs mounts.

Document this in the developer guide, along with the appropriate scary
warnings about using this on a machine that you care about. While we're
in there, also correct a typo about FUSE configuration.

Fixes: https://tracker.ceph.com/issues/41910
Signed-off-by: Jeff Layton <jlayton@redhat.com>