This patch allows to create application containers with liblxc.so directly.

Some code cleanups on the way:
- separate ops for lxc_execute() and lxc_start(): the factorisation is wrong
  here as we may have specific things to do if we're running an application
  container. It deserves separate ops.
- lxc_arguments_dup() is merged in the pre-exec operation: this is a first
  use for the execute op introduced just above. It's better to build the
  arguments to execvp() where they're really used.
Signed-off-by: Greg Kurz <gkurz@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>