Update user addition script to prevent warnings and enhance compatibility

Update user addition script to prevent warnings and enhance compatibility

Not sure if this script is still recommended to use as it's not been modified in nearly a decade.

1. Modified the `useradd` command by removing the `-m` option to prevent unnecessary warnings about the home directory already existing. The creation of the home directory is managed separately in this script using Btrfs subvolumes.

2. Added a command to copy the standard skeleton files from `/etc/skel` to the new user's home directory, ensuring that new users receive the necessary setup files despite the custom home directory creation process with the `create-config` command.

3. Corrected the `chown` command syntax to use the more modern `:` separator between the user and group names, getting rid of another warning.

I kept the (old school) style as is. Thanks for snapper!