Add support for not mounting network-related files

Right now there is no way to stop docker (that I know of, anyhow) from mounting /etc/resolv.conf, /etc/hostname, and /etc/hosts in your containers. These are networking-related files and docker presuming that it knows how to set them in a --net=none environment is flawed. In particular this screws up any kind of orchestration inside the container that needs to move these files, because they are not files and cannot be moved. (This is a common problem for anyone who tries to use dhclient, but any atomic operation that requires making temp files and moving them into place gets broken).

Preferably this would happen if you set --net=none, but I understand that may be a problem for backwards compatibility, so I would be fine with some other switch to flip.

No files, tests, or entry points are named in the issue. Start by locating where /etc/resolv.conf, /etc/hostname, and /etc/hosts are mounted for containers, then determine how a switch or --net=none behavior could disable those mounts without breaking compatibility. Done means users can prevent these mounts and the behavior is covered by tests.