hostifo: update LXC, add Cloud Run.

Recent LXC support no longer has "lxc" in /proc/1/cgroup:
    # cat /proc/1/cgroup
    12:freezer:/
    11:rdma:/
    10:cpuset:/
    9:pids:/
    8:blkio:/
    7:devices:/
    6:perf_event:/
    5:net_cls,net_prio:/
    4:memory:/
    3:hugetlb:/
    2:cpu,cpuacct:/
    1:name=systemd:/init.scope
    0::/init.scope

Look for fuse.lxcfs in /proc.mounts in addition:
    # grep lxc /proc/mounts
    lxcfs /proc/cpuinfo fuse.lxcfs ...
    lxcfs /proc/diskstats fuse.lxcfs ...
    lxcfs /proc/loadavg fuse.lxcfs ...
    lxcfs /proc/meminfo fuse.lxcfs ...
    lxcfs /proc/stat fuse.lxcfs ...
    lxcfs /proc/swaps fuse.lxcfs ...
    lxcfs /proc/uptime fuse.lxcfs ...
    lxcfs /sys/devices/system/cpu/online fuse.lxcfs ...

Add Knative detection by looking for the environment variables
which are part of its container contract.

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

control/controlclient/hostinfo_linux.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"os"
 	"strings"
 	"syscall"
 
@@ -58,6 +59,9 @@ func osVersionLinux() string {
 	if inContainer() {
 		attrBuf.WriteString("; container")
 	}
+	if inKnative() {
+		attrBuf.WriteString("; env=kn")
+	}
 	attr := attrBuf.String()
 
 	id := m["ID"]
@@ -99,5 +103,21 @@ func inContainer() (ret bool) {
 		}
 		return nil
 	})
+	lineread.File("/proc/mounts", func(line []byte) error {
+		if mem.Contains(mem.B(line), mem.S("fuse.lxcfs")) {
+			ret = true
+			return io.EOF
+		}
+		return nil
+	})
 	return
 }
+
+func inKnative() bool {
+	// https://cloud.google.com/run/docs/reference/container-contract#env-vars
+	if os.Getenv("K_REVISION") != "" && os.Getenv("K_CONFIGURATION") != "" &&
+		os.Getenv("K_SERVICE") != "" && os.Getenv("PORT") != "" {
+		return true
+	}
+	return false
+}