erlxc_exec: execute unprivileged commands

Add a binary to exec a command as another user as an alternative to sudo
and start-stop-daemon.

* sudo expects PAM and configuration to exist

* start-stop-daemon requires the UID/GID to exist in /etc/passwd

erlxc_exec can be run by bind mounting it over another executable inside
the container like /sbin/init. For example, to run an unprivileged shell
listening to a port:

    UID="12345",
    GID="12345",
    Port = "12345",
    erlxc:spawn([
            {chroot, [...]},
            {config, [
                {"lxc.mount.entry", "/sbin sbin none ro,bind,nosuid 0 0"},
                {"lxc.mount.entry",
                "/home/msantos/src/erlang/erlxc/priv/erlxc_exec sbin/init none ro,bind 0 0"},
                ...
            ]},
            {start, [
                {argv, "/sbin/init", UID, GID,
                       "/bin/sh", "-c",
                        "/usr/bin/mkfifo
                        /tmp/shell_fifo;"
                        "/bin/cat
                        /tmp/shell_fifo"
                        "| /bin/bash -i 2>&1"
                        "| /bin/nc -l " ++ Port ++ " > /tmp/shell_fifo"
            ]}
    ]).