Build 19018

For general Windows information on build 19018 visit the Windows blog.

• [WSL2] Use cache=mmap as the default for 9p mounts to fix dotnet apps
• [WSL2] Fixes for localhost relay [GH 4340]
• [WSL2] Introduce a cross-distro shared tmpfs mount for sharing state between distros
• Fix restoring persistent network drive for \\wsl$