Update to r52827:

A few changes to the Xen setup:

1. The line sysrc -f /etc/sysctl.conf vm.max_wired=-1 does not work, it only returns an error about a name containing characters not allowed in shell. Instead, use echo to add the line to sysctl.conf.
   1. Fix the rules for automatic bridge creation, taken from the bridging chapter of the handbook.
   2. Add a note that FreeBSD's Dom0 requires booting in legacy (BIOS) mode.
   3. Remove mention of xen.4th for the loader menu. It's not working anymore with the Lua loader that we have now. Until this is fixed, don't refer readers to it.