Fixed user deletion issue in rpm postuninstall scriptlet.

Default rpm postuninstall scriptlet unconditionally deletes user / group.
This is a problem when someone tries to upgrade a package in one step (using rpm -U package-name).
Currently this ends up with upgraded package artifacts and removed package user.

The fix adds a check which tests number of packages which will be left in the system after the action completes:
http://fedoraproject.org/wiki/Packaging:ScriptletSnippets#Syntax