Add fallback mechanism for platforms without attach support

Some platforms (most notably CentOS and RHEL) use a kernel without
'attach' support. This patch detects this absence and falls back
to the alternative ways of doing things like detection of IP address
and halting the container.
It does so by running the command "true" through lxc-attach.
Merge pull request #294 from bkkrw/lxc-attach_support

Add fallback mechanism for platforms without attach support