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>

version/distro/distro.go

@@ -19,6 +19,7 @@ type Distro string
 
 const (
 	Debian    = Distro("debian")
+	Crostini  = Distro("crostini") // ChromeOS Crostini Linux container; Debian-based
 	Arch      = Distro("arch")
 	Synology  = Distro("synology")
 	OpenWrt   = Distro("openwrt")
@@ -84,6 +85,11 @@ func linuxDistro() Distro {
 		// Currently supported product families:
 		// - UDM (UniFi Dream Machine, UDM-Pro)
 		return UBNT
+	case have("/opt/google/cros-containers/bin/garcon"):
+		// ChromeOS Crostini ships /opt/google/cros-containers/bin/garcon
+		// in every penguin container. MUST be checked before Debian since
+		// Crostini is Debian-based.
+		return Crostini
 	case have("/etc/debian_version"):
 		return Debian
 	case have("/etc/arch-release"):