Win32: Use wide strings to parse most command-line arguments.
Also do a tiny bit of cleanup in main.cpp (replacing strcmps with simple string equality checks).
GetWideCmdLine doesn't need to be exposed in the header anymore.
Also fix a warning.
Remove a few unneeded parentheses, and use nullptr instead of NULL.
Merge pull request #6816 from thedax/win32-Unicode

Win32: Use wide strings to parse some command-line arguments, and convert them to proper UTF-8 for others.