We build up maps of both the existing MagicDNS configuration in hosts
and the desired MagicDNS configuration, compare the two, and only
write out a new one if there are changes. The comparison doesn't need
to be perfect, as the occasional false-positive is fine, but this
should greatly reduce rewrites of the hosts file.
I also changed the hosts updating code to remove the CRLF/LF conversion
stuff, and use Fprintf instead of Frintln to let us write those inline.
Updates #14428
Signed-off-by: Aaron Klotz <aaron@tailscale.com>
d818a58a77