Refactor device handling code

We now have one place that keeps track of (most) devices that are allowed and created within the container.  That place is pkg/libcontainer/devices/devices.go

This fixes several inconsistencies between which devices were created in the lxc backend and the native backend.  It also fixes inconsistencies between wich devices were created and which were allowed.  For example, /dev/full was being created but it was not allowed within the cgroup.  It also declares the file modes and permissions of the default devices, rather than copying them from the host.  This is in line with docker's philosphy of not being host dependent.

Docker-DCO-1.1-Signed-off-by: Timothy Hobbs <timothyhobbs@seznam.cz> (github: https://github.com/timthelion)
reuse timestamp, don't call time.Now() 3 times

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #6127 from unclejack/reuse_timestamp

reuse timestamp, don't call time.Now() 3 times
Merge pull request #6097 from timthelion/consistentdevices

Refactor device handling code