sys-boot/systemrescuecd-x86-grub: Update for 6

sys-boot/systemrescuecd-x86-grub: Update for 6

SystemRescueCD starting from version 6 is based on Arch and the way it
is booted changed.

Rewrite to:
- Support SystemRescueCD versions 5 and 6.

- Create menu entries for all versions found.

- Add option to copy the newest ISO to /boot if there is enough free
space. Helpful if / becomes unaccessible while /boot is on a separate
partition.

- Add a menu entry for persistent boot (version 6 only). Will put data
on the same disk, where the current ISO resides, in
"/persistent_sysresccd-${version}/x86_64".

Signed-off-by: Reinis Danne <rei4dan@gmail.com>
Signed-off-by: Michał Górny <mgorny@gentoo.org>