archiso/mkarchiso: reduce duplication and add more info messages

Fixes to issues introduced in !59 :
- _make_boot_on_fat(): copy initramfs from "${airootfs_dir}/boot/" not "${isofs_dir}/". Otherwise UEFI-only ISOs cannot be built.

Some general fixes:
- Replace mkdir with install. Unlike mkdir, install does not complain when the target exists.
- Reduce excess newlines produced by messages.
- Ensure FAT image gets unmounted in case the script is interrupted.
- Create the ext4 image with mkfs.ext4 instead of truncate.
- Do not rely on user and group names for chown commands. Use numeric UID and GID instead.
- Minimize the times stderr is redirected to /dev/null.
- Add missing '?' to getopts.
- Standardize function definitions by removing spaces between the function name and () .
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