Remove some overhead from Process.Start (#44691)

Remove some overhead from Process.Start (#44691)

- Avoid StringBuilder marshaling
- Let CreateProcess{WithLogonW} determine the current working directory
- Avoid creating SafeHandles for GetCurrentProcess()
- Avoid forcing ProcessStartInfo.ArgumentList into existence just to check if it contains anything
- Prefer using ProcessStartInfo.Arguments in Start(string, IEnumerable<string>) if there's only one string in the enumerable
- Use ValueStringBuilder instead of StringBuilder to build up arguments, so as to use stack space / pooled char[]s and avoid actually needing to produce strings.
- Avoid unnecessary SafeHandle for PROCESS_INFORMATION.hThread