Support for Firewalld

Firewalld [1] is a firewall managing daemon with D-Bus interface.

What sort of problem are we trying to solve with this ?

Firewalld internally also executes iptables/ip6tables to change firewall settings.
It might happen on systems where both docker and firewalld are running
concurrently, that both of them try to call iptables at the same time.
The result is that the second one fails because the first one is holding a xtables lock.
One workaround is to use --wait/-w option in both
docker & firewalld when calling iptables.
It's been already done in both upstreams:
https://github.com/docker/docker/commit/b315c380f4acd65cc0428009702f99a266f96c59
https://git.fedorahosted.org/cgit/firewalld.git/commit/?id=b3b451d6f8946986b8f50c8bcddeef50ed7a5f8f
But it'd still be better if docker used firewalld when it's running.

Other problem the firewalld support would solve is that
iptables/firewalld service's restart flushes all firewall rules
previously added by docker.
See next patch for possible solution.

This patch utilizes firewalld's D-Bus interface.
If firewalld is running, we call direct.passthrough() [2] method instead
of executing iptables directly.
direct.passthrough() takes the same arguments as iptables tool itself
and passes them through to iptables tool.
It might be better to use other methods, like direct.addChain and
direct.addRule [3] so it'd be more intergrated with firewalld, but
that'd make the patch much bigger.
If firewalld is not running, everything works as before.

[1] http://www.firewalld.org/
[2] https://jpopelka.fedorapeople.org/firewalld/doc/firewalld.dbus.html#FirewallD1.direct.Methods.passthrough
[3] https://jpopelka.fedorapeople.org/firewalld/doc/firewalld.dbus.html#FirewallD1.direct.Methods.addChain
    https://jpopelka.fedorapeople.org/firewalld/doc/firewalld.dbus.html#FirewallD1.direct.Methods.addRule
React to firewalld's reload/restart.

When firewalld (or iptables service) restarts/reloads,
all previously added docker firewall rules are flushed.

With firewalld we can react to its Reloaded() [1] D-Bus signal and recreate the firewall rules.
Also when firewalld gets restarted (stopped & started) we can catch the NameOwnerChanged signal [2].
To specify which signals we want to react to we use AddMatch [3].

Libvirt has been doing this for quite a long time now.

Docker changes firewall rules on basically 3 places.
1) daemon/networkdriver/portmapper/mapper.go - port mappings
   Portmapper fortunatelly keeps list of mapped ports,
   so we can easily recreate firewall rules on firewalld restart/reload
   New ReMapAll() function does that.
2) daemon/networkdriver/bridge/driver.go
   When setting a bridge, basic firewall rules are created.
   This is done at once during start, it's parametrized and nowhere
   tracked so how can one know what and how to set it again when
   there's been firewalld restart/reload ?
   The only solution that came to my mind is using of closures [4],
   i.e. I keep list of references to closures (anonymous functions
   together with a referencing environment) and when there's firewalld
   restart/reload I re-call them in the same order.
3) links/links.go - linking containers
   Link is added in Enable() and removed in Disable().
   In Enable() we add a callback function, which creates the link, that's OK so far.
   It'd be ideal if we could remove the same function from
   the list in Disable(). Unfortunatelly that's not possible AFAICT,
   because we don't know the reference to that function
   at that moment, so we can only add a reference to function,
   which removes the link. That means that after creating and
   removing a link there are 2 functions in the list,
   one adding and one removing the link and after
   firewalld restart/reload both are called.
   It works, but it's far from ideal.

[1] https://jpopelka.fedorapeople.org/firewalld/doc/firewalld.dbus.html#FirewallD1.Signals.Reloaded
[2] http://dbus.freedesktop.org/doc/dbus-specification.html#bus-messages-name-owner-changed
[3] http://dbus.freedesktop.org/doc/dbus-specification.html#message-bus-routing-match-rules
[4] https://en.wikipedia.org/wiki/Closure_%28computer_programming%29