ssh/tailssh: work around lack of scontext in SELinux

Trying to SSH when SELinux is enforced results in errors like:

```
➜  ~ ssh ec2-user@<ip>
Last login: Thu Jun  1 22:51:44 from <ip2>
ec2-user: no shell: Permission denied
Connection to <ip> closed.
```

while the `/var/log/audit/audit.log` has
```
type=AVC msg=audit(1685661291.067:465): avc:  denied  { transition } for  pid=5296 comm="login" path="/usr/bin/bash" dev="nvme0n1p1" ino=2564 scontext=system_u:system_r:unconfined_service_t:s0 tcontext=unconfined_u:unconfined_r:unconfined_t:s0 tclass=process permissive=0
```

The right fix here would be to somehow install the appropriate context when
tailscale is installed on host, but until we figure out a way to do that
stop using the `login` cmd in these situations.

Updates #4908

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnlocal/local.go

@@ -4,7 +4,6 @@
 package ipnlocal
 
 import (
-	"bytes"
 	"context"
 	"encoding/base64"
 	"encoding/json"
@@ -18,7 +17,6 @@ import (
 	"net/netip"
 	"net/url"
 	"os"
-	"os/exec"
 	"os/user"
 	"path/filepath"
 	"runtime"
@@ -2583,7 +2581,7 @@ func (b *LocalBackend) checkSSHPrefsLocked(p *ipn.Prefs) error {
 		if distro.Get() == distro.QNAP && !envknob.UseWIPCode() {
 			return errors.New("The Tailscale SSH server does not run on QNAP.")
 		}
-		checkSELinux()
+		b.updateSELinuxHealthWarning()
 		// otherwise okay
 	case "darwin":
 		// okay only in tailscaled mode for now.
@@ -4705,12 +4703,8 @@ func (b *LocalBackend) sshServerOrInit() (_ SSHServer, err error) {
 
 var warnSSHSELinux = health.NewWarnable()
 
-func checkSELinux() {
-	if runtime.GOOS != "linux" {
-		return
-	}
-	out, _ := exec.Command("getenforce").Output()
-	if string(bytes.TrimSpace(out)) == "Enforcing" {
+func (b *LocalBackend) updateSELinuxHealthWarning() {
+	if hostinfo.IsSELinuxEnforcing() {
 		warnSSHSELinux.Set(errors.New("SELinux is enabled; Tailscale SSH may not work. See https://tailscale.com/s/ssh-selinux"))
 	} else {
 		warnSSHSELinux.Set(nil)
@@ -4722,7 +4716,7 @@ func (b *LocalBackend) handleSSHConn(c net.Conn) (err error) {
 	if err != nil {
 		return err
 	}
-	checkSELinux()
+	b.updateSELinuxHealthWarning()
 	return s.HandleSSHConn(c)
 }