conf: ret-try devpts mount without gid=5 on error
conf: ret-try devpts mount without gid=5 on error

We should always default to mounting devpts with gid=5 but we should fallback
to mounting without gid=5. This let's us cover use-cases such as container
started with only a single mapping e.g.:

lxc.idmap = u 1000 1000 1
lxc.idmap = g 1000 1000 1

Closes #2257.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

ss1h2a3tw/lxc,caglar10ur/lxc,hallyn/lxc,ss1h2a3tw/lxc,hallyn/lxc,hallyn/lxc,caglar10ur/lxc,ss1h2a3tw/lxc,caglar10ur/lxc