Add support for >4GB ISO by optionally supporting a second partition. Plus other small improvements.

[cshandley-uk]

Like many people, I've been desperately looking for something to replace Ventoy (due to it's huge number of mystery binary blobs, opaque build process & foreign origin), and GLIM came really close - it just didn't support ISOs >4GB. I've been able to solve that major limitation by adding support for a second partition that uses (say) Ext4. I've tried to keep to the style of your source code, made my changes as small as possible, and split the changes into small commits so you easily see what I've done & why.

I have also made some other small improvements which I hope you won't object to:

• Detect if GPT (rather than MBR) is used, and warn that GRUB requires an extra BIOS Boot Partition for the non-EFI case.
• Made BIOS installation optional if you pick EFI, in case you don't have a BIOS Boot Partition.
• When using a single partition, the ISO folder has been moved from boot/iso/ to just iso/, so it's in the same location whether you use one or two partitions (simplifies documentation & code), as well as IMHO being easier for newbies to find.
• Made "Ready to install GLIM" default to "No", so you can't accidentally proceed if you are just pressing enter without reading the prompts.
• Made the Y/N prompts case insensitive.
• Warn rather than error if there are more than 2 partitions on the block device. (My use case is having bootable ISOs on my external backup HD, so I can boot & restore all from one device - Ventoy allows that sort of thing. Plus the BIOS Boot Partition.)
• Halt if there are multiple GLIM partitions, since it seems asking for trouble if just randomly pick one!
• Added quoting so it can handle the spaces in $USBMNT, just in case of weird systems.

I've tested all these changes as much as I can, on a real physical machine (no VM).