netapi32: Improve NetUserGetInfo function

[Joris Huizer]

Simon Kissane wrote:
> netapi32: Improve NetUserGetInfo function
> * Implemented support for all levels documented by MSDN
> * Corrected error codes for nonexistent levels
> * Now works with empty string as server name
> 
> I have also attached a C program (which can be compiled with MINGW),
> which tests the NetUserGetInfo functionality.
> 

Perhaps you could write unit tests based on the test program, to show 
correctness of the current code - this helps avoiding regressions

HTH,

Joris