improve reform-boot-config, reform-display-config and reform-migrate

• sbin/reform-migrate: reuse reform-boot-config to reduce code duplication
• sbin/reform-setup-encrypted-nvme: use same crypttab options as debian-installer
• sbin/reform-setup-encrypted-nvme: offer to run write resume, crypttab and fstab and run reform-migrate
• sbin/reform-boot-config,reform-migrate: do not overwrite swap settings
• sbin/reform-display-config: use findmnt instead of grepping mount output
• sbin/reform-boot-config: allow operating on rootfs other than the currently running one
• sbin/reform-migrate: use here-document to prevent globbing and word splitting of $TARGET
• sbin/reform-migrate: prefixing with x is no longer necessary since bash 2.0 released in 1996
• sbin/reform-migrate: abort early if something is mounted on /mnt already
• sbin/reform-migrate: add instructions how to partition NVMe