* 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().
