Bug 251584

Description of problem:
Using the properties dialog it is possible to override an existing user when
modifying the name of a different user.

Version-Release number of selected component (if applicable):
1.2.60

How reproducible:
Every time

Steps to Reproduce:
1.As a none root user open the system-config-user utility - enter root password
when prompted.
2.Select any none root user from the list, in my case the user is called ray.
3.Click the properties button.
4.Alter the username field to root and press ok

  
Actual results:
The user gets renamed to root.  Additionally if you inspect /etc/passwd you will
see that it now contains two entries for root.  Each entry in passwd has been
assigned the user id of the selected user (in step 2).  Also, when rebooting the
system struggles to start and reports several warnings, which relate to the root
user permissions.

Expected results:
The utility should inform the user that the name is already being used and force
them to select a unique name.  

Additional info:
The create user function already checks for the existence of an existing user
with the same name.