* 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().