fixed create_user fullname and added createhome option

In options to useradd: replaced --gecos with -c for fullname.
On all OS that I have access to (Ubuntu, Debian, CentOS, Gentoo) the correct option is -c

Also added a createhome option at the end of args that defaults to True.  This preserves current functionality, but now makes it possible to create service users without homedirs.