unexpanded config file: turn into a string

Originally, we only kept a struct lxc_conf representing the current
container configuration.  This was insufficient because lxc.include's
were expanded, so a clone or a snapshot would contain the expanded
include file contents, rather than the original "lxc.include".  If
the host's include files are updated, clones and snapshots would not
inherit those updates.

To address this, we originally added a lxc_unexp_conf, which mirrored
the lxc_conf, except that lxc.include was not expanded.

This has its own cshortcomings, however,  In particular, if a lxc.include
has a lxc.cgroup setting, and you use the api to say:

c.clear_config_item("lxc.cgroup")

this is not representable in the lxc_unexp_conf.  (The original problem,
which was pointed out to me by stgraber, was slightly different, but
unlike this problem it was not unsolvable).

This patch changes the unexpanded configuration  to be a textual
representation of the configuration.  This allows us *order* the
configuration commands, which is what was not possible using the
struct lxc_conf *lxc_unexp_conf.

The write_config() now becomes a simple fwrite.  However, lxc_clone
is slightly complicated in parts, the worst of which is the need to
rewrite the network configuration if we are changing the macaddrs.

With this patch, lxc-clone and clear_config_item do the right thing.
lxc-test-saveconfig and lxc-test-clonetest both pass.

There is room for improvement - multiple calls to

c.append_config_item("lxc.network.link", "lxcbr0")

will result in multiple such lines in the configuration file.  In that
particular case it is harmless.  There may be cases where it is not.

Overall, this should be a huge improvement in terms of correctness.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>