Correct argument names in usage text.

Also try to avoid confusion with user/username/name by using «identifier».
Test if user exists before trying to create it

If so, guess user homedir instead of hardcoding to /home
Use @mikeyxstudios workaround to fix progrium/sshcommand#5
Rework getent test to work with set -e
Simplified readme
Create LICENSE
Let the system decide where a user's home directory should live