proxmox-boot: add pin/unpin functionality for non-p-b-t systems

proxmox-boot: add pin/unpin functionality for non-p-b-t systems

While running `update-grub` directly in this case is a divergence from
the semantics of the command when p-b-t handles booting it makes the
cleanup in the `next-boot` case a bit tidier.

fetching the next-boot version explicitly again before setting the
provided version is to cover the sequence:
p-b-t kernel pin <ver1> --next-boot ; p-b-t kernel pin <ver2>

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Tested-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>