Move choose_init into utils.c so we can re-use it.  Make it and on_path
accept an optional rootfs argument to prepend to the paths when checking
whether the file exists.

Also add lxc.init.static to .gitignore
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>