hammer expand of root fs

[Jan Lentfer]

Today I expanded my root hammer fs by a second device. The expand went 
flawless and except for some bogus numbers in df everything was fine.

Problems began when I rebooted the machine: I ended up in a unusable 
system. I could not get mountroot to use the multi device hammer fs.

So we should either have (sorted by ascending coolness):

1) have a FAT warning in hammer man page to not expand the root file system

2) have hammer expand check and refuse to expand the root file system

3) have a mountroot that can handle multi device hammer fs roots.

Jan