lxc.id_map bug when writing directly to /proc/pid/[ug]id_map

There's some code in src/lxc/conf.c that sets up the UID/GID mapping. It
can use the external newuidmap/newgidmap tools, or it can write to
/proc/pid/[ug]id_map directly. The latter case is broken: lines are written
without a newline (\n) at the end. This patch fixes that. Note that
I did not check if the newuidmap/newgidmap case still works. It should,
but I wasn't able to test it.
Signed-off-by: Miquel van Smoorenburg <mikevs@xs4all.net>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>