Rename usrmgr to nusrmgr

Currently, the user control panel applet is called usrmgr. In Windows XP, it's called nusrmgr. To maintain compatibility, we should change it. CORE-9047 had to change it to usrmgr instead of nusrmgr but it should be changed so apps can launch it if needed.

https://support.microsoft.com/en-us/kb/313808