lxc_abstract_unix_connect: accomodate containers started before Oct 28

commit aae93dd3dd20dd12c6b8f9f0490e2fb877ee3f09 fixed the command socket
name to use the right pathlen instead of always passing in the max
socket namelen.  However, this breaks lxc-info/lxc-list/etc for
containers started before that commit.  So if the correct command
sock name doesn't work, try the preexising one.

Note we can probably undo this "after awhile".  Maybe in august 2014.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>