Previously tailscaled on macOS was running "/sbin/route monitor" as a
child process, but child processes aren't allowed in the Network
Extension / App Store sandbox. Instead, just do what "/sbin/route monitor"
itself does: unix.Socket(unix.AF_ROUTE, unix.SOCK_RAW, 0) and read that.
We also parse it now, but don't do anything with the parsed results yet.
We will over time, as we have with Linux netlink messages over time.
Currently any message is considered a signal to poll and see what changed.
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Tangentially related to #987, #177, #594, #925, #505
Motivated by rebooting a launchd-controlled tailscaled and it going
into SetNetworkUp(false) mode immediately because there really is no
network up at system boot, but then it got stuck in that paused state
forever, without a monitor implementation.
And make the monitor package portable with no-op implementations on
unsupported operating systems.
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
