Btrfs tweak and better MBR support for cgpt

The first commit simply allows resizing the partition table even if the filesystem itself isn't supported, this is the case with btrfs right now. Later an online resize can be performed on btrfs instead.

The rest of this adds real support for writing MBR partition tables to cgpt. Previously the protective MBR was only written when writing the bootloader code to disk with `cgpt boot` but not when creating, resizing, or repairing the GPT. Now that is always done. Support for hybrid MBRs and the "Legacy BIOS Bootable" flag has been added which fixes compatibility with pvgrub and newer versions of SYSLINUX.