installer: Raise the size proposal for /boot to 768M for HAMMER installs.

installer: Raise the size proposal for /boot to 768M for HAMMER installs.

Rewrite the algorithm for calculating size recommendations to be a bit
more flexible, especially with smaller disks (as in VMs e.g.). It should
always give a valid set now.

/boot tries to be 768M if space permits. This way several kernels and
module directories can comfortably fit on it.

Reported-by: many
While I'm here, fix another issue regarding HAMMER installs:

The check for filesystems <50GB wasn't working at all. Fix and improve it.
Instead of checking only the root filesystem, check the sizes of all HAMMER
filesystems created. If a HAMMER filesystem <50GB has been encountered,
display a warning similar to hammer_newfs(8) and allow the user to either
confirm or cancel.