cmd/tailscaled, version/distro: default to userspace-networking on Crostini

cros-garcon NULL-derefs on cold-boot netlink enumeration when
tailscale0 is present, preventing the Crostini container and
ChromeOS Terminal from starting cleanly. This is an upstream
ChromiumOS bug in cros-garcon; tailscaled can work around it
by defaulting to userspace-networking mode on Crostini.

Tailscale SSH continues to work via tailscaled's netstack.
Users can override with --tun=tailscale0 on ChromeOS builds
where cros-garcon is fixed.

Crostini is detected via /opt/google/cros-containers/bin/garcon,
which is present in every Crostini penguin container.

ssh/tailssh extends the existing Debian default-PATH case to
cover Crostini, since Crostini is Debian-based and benefits
from the same SSH PATH defaults.

RELNOTE: Crostini now defaults to userspace-networking.

Fixes #19488
Updates #12090

Signed-off-by: ferrumclaudepilgrim <ferrumclaudepilgrim@users.noreply.github.com>

cmd/tailscaled/tailscaled.go

@@ -87,6 +87,16 @@ func defaultTunName() string {
 			// See https://github.com/tailscale/tailscale-synology/issues/35
 			return "tailscale0,userspace-networking"
 		}
+		if buildfeatures.HasNetstack && distro.Get() == distro.Crostini {
+			// cros-garcon NULL-derefs on cold-boot netlink interface
+			// enumeration when tailscale0 is present, preventing the
+			// Crostini container and ChromeOS Terminal from starting
+			// cleanly. Default to userspace-networking until the
+			// upstream ChromiumOS bug is fixed.
+			// See https://github.com/tailscale/tailscale/issues/12090
+			// See https://issuetracker.google.com/issues/517069318
+			return "userspace-networking"
+		}
 	}
 	return "tailscale0"
 }