Some editions of Windows server share the same build number as their
client counterpart; we must use an additional field found in the OS
version information to distinguish between them.
Even though "Distro" has Linux connotations, it is the most appropriate
hostinfo field. What is Windows Server if not an alternate distribution
of Windows? This PR populates Distro with "Server" when applicable.
Fixes#11785
Signed-off-by: Aaron Klotz <aaron@tailscale.com>