Updated YubiKey packages, added sudo, custom user

All YubiKey packages have been updated to the latest available
versions.

The script now prompts for a username to attach the YubiKey to,
creates the user if it does not exist, creates the yubikey group,
and adds the specified user to that group. This allows YubiKey
authentication to be enabled on a per-user basis rather than
system-wide. Enabling YubiKey authentication system-wide
effectively disables all users that are not configured with a
valid YubiKey.

Then, the script adds the specified user with ALL privileges
to /etc/sudoers.d/yubikey and disables root logins via SSH.
Updated permissions, minor tweaks to logic

Updated permissions of /etc/yubikey storage file to 440 and optimized
an if statement.