Bugfix to support unbound keyslot when sealing key

[thedafidev]

Fix for #2045

Changes introduced:

• unbound keyslots are now ignored (thanks to a new regex and sed command in kexec-seal-key)
• if by any chance luksDump parsing founds non-numeric keyslot numbers, they get ignored

It's the first time I change heads source code so I'm not sure I did everything as I should

I tested the new regex and sed command against some luksDump outputs with the following command:

cat luks.txt | grep -E "^\s+([0-9]+):\s*luks2$" | sed "s/^\s\+\([0-9]\+\):\s*luks2$/\1/g"