[netshell] Implement NcIsValidConnectionName

Documentation: https://msdn.microsoft.com/en-us/library/windows/desktop/aa366197%28v=vs.85%29.aspx

Implement NcIsValidConnectionName. It's one of the 2 documented functions on MSDN for netshell. Might as well implement it as HrRenameConnection is going to use this function. Big thanks to encoded and hbelusca for helping me with this. I hope everything is fine with the code and the patches. I have never had much luck with Tortoise SVN working right with creating new folders via patches.