mkarchiso: support setting gpg sender

nl6720requested to merge
nl6720/archiso:gpg-sender into master

Add new -G option to set gpg's --sender. This allows to see who signed the rootfs image without needing to import the gpg key from the keyring in initramfs.

I plan to create a MR for mkinitcpio/mkinitcpio-archiso> to show the relevant part of the gpg --verify output (the signature and thus also sender) when verify is set in the kernel command line.

Edited by nl6720

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