* qemud/qemud.c, qemud/bridge.c, qemud/iptables.c: fix
our FD_CLOEXEC usage so that all fds which should be
closed on exec are marked as such and that we leave
exec() to do the actual closing.
* configure.in: add --disable-bridge-params, check
for libsysfs and various kernel headers
* bridge.[ch]: add code for managing bridges
* qemud/Makefile.am: add bridge.[ch] and link against
libsysfs if enabled.
* qemud/conf.c: add support for bridge config.
* qemud/internal.h: add various bridging bits
* qemud/qemud.c: implement qemudStartNetworkDaemon()
and qemudShutdownNetworkDaemon().
