Add support for users on windows

[johnscillieri]

I have a tough time testing this one, I don't really have a multi-user server with remote connections to test against.

Seems to work:

C:\Users\John\Documents\workspace\psutil-nim> nim c -d:release -r .\tests\quick_test.nim
Hint: used config file 'C:\nim\config\nim.cfg' [Conf]
Hint: operation successful (300 lines compiled; 0.068 sec total; 5.754MiB peakmem; Release Build) [SuccessX]
Hint: C:\Users\John\Documents\workspace\psutil-nim\tests\quick_test.exe  [Exec]

users()
@[(name: "John", terminal: "", host: "0.0.0.0", started: 1571358623.800257)]