podman machine: make 9p security model configurable; adjust docs

This addresses:
Symlinks don't work on podman machine on macOS Monterey when using volumes feature #13784

This change does NOT exactly fix the bug, but it does allow the user to
work around it via 'podman init' option, e.g.:
podman machine init -v "$HOME/git:$HOME/git:ro:security_model=none"

If the default security model were to be changed to 'none', then that
would fix the bug, at the possible cost of breaking any use cases that
depend on 'mapped-xattr'.

The documentation of the purpose and behavior of the different security
models seems to be rather light:
https://wiki.qemu.org/Documentation/9psetup#Starting_the_Guest_directly

From testing, it appears that the mapped-xattr security model intends to
manage symlinks such that the guest can see the symlinks but the host
only sees regular files (with extended attributes). As far as I can
tell, this behavior only makes sense when the guest is the only thing
that ever needs to create and read symlinks. Otherwise, symlinks created
on the host are unusable on the guest, and vice versa.

As per the original commit: 8e7eeaa4dd14621bda15e396fcd7b9187bc500c5
[NO NEW TESTS NEEDED]

Also document existing ro and rw options.

Also remove misleading statement about /mnt. By my observation, this
line is incorrect. If the intended meaning is different, then I don't
understand.

The default volume is mounted read/write and is not within /mnt.

[core@localhost ~]$ mount | grep 9p
vol0 on /Users/chickey type 9p (rw,relatime,sync,dirsync,access=client,trans=virtio)

Signed-off-by: Corey Hickey <chickey@tagged.com>

0 files changed, 0 insertions, 0 deletions