lxc-start-ephemeral: handle the overlayfs workdir option (v2)

lxc-start-ephemeral: handle the overlayfs workdir option (v2)

We fixed this some time ago for basic lxc-start, but never did
lxc-start-ephemeral.

Since the lxc-start patches were pushed, Miklos has given us a
way to detect whether we need the workdir= option.  So the
bdev.c code could be simplified to check for "overlay\n" in
/proc/filesystems just as lxc-start-ephemeral does.  This
patch doesn't do that.

Changelog (v2):
	1. use 'overlay' fstype for new overlay upstream module
	2. avoid using unneeded readlines().

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>