LVM growing and bootloader, again.

[David Lehman <dlehman redhat com>]

Looking forward to GRUB2 on ppc made me rethink which bits of
data are really bootloader-specific as opposed to being
platform-specific. It turns out that all of the stage1
requirements are really per-platform and will remain
constant even when moving from, say, yaboot to grub2.

I noticed the relative sizes of root and /home weren't ideal
for disks only slightly larger than the 50G minimum to get
a separate /home. In the course of testing different values
I encountered some cases of a single leftover PE after growing
LVM. I decided that growing in units of PE size instead of MB
eliminates the need to worry about aligning to PE size, and is
therefore a cleaner approach.