archiso/mkarchiso: various fixes

Fixes to issues introduced in !59 (merged) :

• _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 ().