kernel config - Build 'vn' into the kernel

* Build vn into the kernel, we are using it in the build system and
  its not a good idea to kldload it during a buildworld or installworld.
build - separate out the rescue initrd

* Multiple issues with users crashing at the end of an installworld because
  initrd kldloads an out-of-date vn device.

* Not a good idea to update the rescue image during an installworld anyway.

* Make it a manual target called 'rescue' and remind the user at the end
  of the installworld that the target is available, but suggest
  rebooting first.
moused - Ease usb mouse attach/detach

WARNING: Upgrading will overwrite your /etc/devd.conf.

Extend /etc/rc.d/moused. To attach e.g. ums0, you can now call:

    /etc/rc.d/moused start ums0

To detach:

    /etc/rc.d/moused stop ums0

If moused is called with an explicit device name, it is enabled by default
(moused_nondefault_enable="YES").

You can explicitly override any settings in rc.conf with directives like:

    moused_ums0_enable="YES"
    moused_ums0_type="..."

If you have one or more ums[0-9]+ mouses, you can now make use of them by
simply setting devd_enable="YES" in rc.conf.