Introduce --lxcpath cmdline option, and make default_lxc_path() return const char *

For the lxc-* C binaries, introduce a -P|--lxcpath command line option
to override the system default.

With this, I can

    lxc-create -t ubuntu -n r1
    lxc-create -t ubuntu -n r1 -P /home/ubuntu/lxcbase
    lxc-start -n r1 -d
    lxc-start -n r1 -d -P /home/ubuntu/lxcbase
    lxc-console -n r1 -d -P /home/ubuntu/lxcbase
    lxc-stop -n r1

all working with the right containers (module cgroup stuff).

To do:
    * lxc monitor needs to be made to handle cgroups.
      This is another very invasive one.  I started doing this as
      a part of this set, but that gets hairy, so I'm sending this
      separately.  Note that lxc-wait and lxc-monitor don't work
      without this, and there may be niggles in what I said works
      above - since start.c is doing lxc_monitor_send_state etc
      to the shared abstract unix domain socket.
    * Need to handle the cgroup conflicts.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>