attach: Support unprivileged containers

This change makes lxc-attach and the matching API functions work
properly with unprivileged containers.

The trick needed to make that possible was to always start with the
userns when attaching and also relocate the cgroup management code so
that the intermediate process is moved to the cgroup before attaching to
the container's namespace as doing so later would fail due to missing
permissions.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
utils: Drop trailing / in lxcpath

This fixes command line tools and functions which use
lxc_global_config_value o get lxcpath but don't strip the trailing /
leading to mismatching command path (as lxc_container_new does strip the
path).

As lxcpath is typically a const and so can't easily be changed by the
caller, add the trick directly into lxc_global_config_value (having to
juggle a bit in there too to avoid trying to alter a const).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Move lxc-user-nic and lxc-monitord to libexec

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>