Add a container config option to mount and populate /dev in a container.

We might want to add options to specify a max size for /dev other than
the default 100k, and to specify other devices to create.  And maybe
someone can think of a better name than autodev.

Changelog: Don't error out if we couldn't mknod a /dev/ttyN.
Changelog: Describe the option in lxc.conf manpage.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>