gptboot rewrite, bootonce, etc.

[Andrey Fesenko]

This work if use ZFS?
My issues "Root on ZFS & GPT and boot to ufs partition"
http://lists.freebsd.org/pipermail/freebsd-fs/2012-January/013514.html

I test
# gpart show
=>       34  625142381  ada0  GPT  (298G)
         34        128     1  freebsd-boot  (64k)
        162   26621952     2  freebsd-ufs  [bootonce,bootme]  (12G)
   26622114    8388608     3  freebsd-swap  (4.0G)
   35010722  590131693     4  freebsd-zfs  (281G)

system ada0p2 not boot.