Make lxc@.service forking

Previously the init process' output was dumped into the log
files since the service used Type=simple and
StandardOutput/Error=syslog.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

fix segfault in lxc-attach

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

CVE-2017-5985: Ensure target netns is caller-owned

Before this commit, lxc-user-nic could potentially have been tricked into
operating on a network namespace over which the caller did not hold privilege.

This commit ensures that the caller is privileged over the network namespace by
temporarily dropping privilege.

Launchpad: https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1654676
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

possibility to run lxc-monitord as a regular daemon

This includes an lxc-monitord.service, required by
lxc@.service which is now of Type=forking.

Previously the init process' output was dumped into the log
files since the service used Type=simple and
StandardOutput/Error=syslog. Using lxc-start's daemon mode
on the other hand used a wait call spawning an lxc-monitord
in the background which could potentially stick around
forever if there were clients connected to it. Since it was
considered part of the lxc@foo.service unit by systemd this
also meant the unit was considered active until not only the
container but also lxc-monitord exited.
This is now corrected by creating a separate lxc-monitord
unit which lxc@.service depends on.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>