windows: support launching executables with runas permissions

[vtjnash]

Calling UAC applications requires a complicated, undocumented RPC call, or using the runas verb to ShellExecuteEx to do that for the program. Perhaps this should be a flag to uv_spawn, so that users can run processes such as regedit.exe and setup.exe (after clicking on the UAC prompt) with libuv, without requiring workarounds such as cmd /c regedit, which will run the ShellExecute function verb "open"? See https://learn.microsoft.com/en-us/windows/win32/shell/launch

[CGQAQ]

I'll take this one, BTW, ShellExecuteEx doesn't support some of the args of CreateProcessW, should we just ignore related flags silently? and also ShellExecuteEx only gives back process handle, I think it's not trivial to get the primary thread handle from the process handle

[vtjnash]

Figuring out what flags are mapped or which are disallowed would be necessary, yes