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>

hostinfo/hostinfo.go

@@ -7,8 +7,10 @@ package hostinfo
 
 import (
 	"bufio"
+	"bytes"
 	"io"
 	"os"
+	"os/exec"
 	"runtime"
 	"runtime/debug"
 	"strings"
@@ -434,3 +436,12 @@ func etcAptSourceFileIsDisabled(r io.Reader) bool {
 	}
 	return disabled
 }
+
+// IsSELinuxEnforcing reports whether SELinux is in "Enforcing" mode.
+func IsSELinuxEnforcing() bool {
+	if runtime.GOOS != "linux" {
+		return false
+	}
+	out, _ := exec.Command("getenforce").Output()
+	return string(bytes.TrimSpace(out)) == "Enforcing"
+}