From: Daniel Lezcano <dlezcano@fr.ibm.com>

The main difference between lxc_start and lxc_execute is the latter creates
an intermediate process to wait for all the childs. That allows to support
daemons or orphan process group for the pid namespace.

Having such difference makes the code to be duplicate between the two
functions. So instead of doing this, I create an intermediate <init> program
which is in charge to launch the specified command. This command is the
lxc-init program taking different options:
 --mount-procfs : mount the proc filesystem before exec'ing the command
 --mount-sysfs : mount the sys filesystem before exec'ing the command

A double dash indicates the end of the options of lxc-init and the beginning
of the command to be launched.

To summarize:
 * lxc_execute function is no more.
 * lxc-execute command uses the lxc_start function and launch the specified
command via lxc-init
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>