feat: emit explicit errors for the `service` command on unsupported OSes

Per the contribution guidelines, this seemed to me like a small enough
change to not warrant an issue before creating this pull request. Let me
know if you'd like me to create one anyway.

## Background

While working with `cloudflared` on FreeBSD recently, I noticed that
there's an inconsistency with the available CLI commands on that OS
versus others — namely that the `service` command doesn't exist at all
for operating systems other than Linux, macOS, and Windows.

Contrast `cloudflared --help` output on macOS versus FreeBSD (truncated
to focus on the `COMMANDS` section):

- Current help output on macOS:

  ```text
  COMMANDS:
     update     Update the agent if a new version exists
     version    Print the version
     proxy-dns  Run a DNS over HTTPS proxy server.
     tail       Stream logs from a remote cloudflared
     service    Manages the cloudflared launch agent
     help, h    Shows a list of commands or help for one command
     Access:
       access, forward  access <subcommand>
     Tunnel:
tunnel Use Cloudflare Tunnel to expose private services to the Internet
or to Cloudflare connected private users.
  ```
- Current help output on FreeBSD:
  ```text
  COMMANDS:
     update     Update the agent if a new version exists
     version    Print the version
     proxy-dns  Run a DNS over HTTPS proxy server.
     tail       Stream logs from a remote cloudflared
     help, h    Shows a list of commands or help for one command
     Access:
       access, forward  access <subcommand>
     Tunnel:
tunnel Use Cloudflare Tunnel to expose private services to the Internet
or to Cloudflare connected private users.
  ```

This omission has caused confusion for users (including me), especially
since the provided command in the Cloudflare Zero Trust dashboard
returns a seemingly-unrelated error message:

```console
$ sudo cloudflared service install ...
You did not specify any valid additional argument to the cloudflared tunnel command.

If you are trying to run a Quick Tunnel then you need to explicitly pass the --url flag.
Eg. cloudflared tunnel --url localhost:8080/.

Please note that Quick Tunnels are meant to be ephemeral and should only be used for testing purposes.
For production usage, we recommend creating Named Tunnels. (https://developers.cloudflare.com/cloudflare-one/connections/connect-apps/install-and-setup/tunnel-guide/)
```

## Contribution

This pull request adds a "stub" `service` command (including the usual
subcommands available on other OSes) to explicitly declare it as
unsupported on the operating system.

New help output on FreeBSD (and other operating systems where service
management is unsupported):

```text
COMMANDS:
   update     Update the agent if a new version exists
   version    Print the version
   proxy-dns  Run a DNS over HTTPS proxy server.
   tail       Stream logs from a remote cloudflared
   service    Manages the cloudflared system service (not supported on this operating system)
   help, h    Shows a list of commands or help for one command
   Access:
     access, forward  access <subcommand>
   Tunnel:
     tunnel  Use Cloudflare Tunnel to expose private services to the Internet or to   Cloudflare connected private users.
```

New outputs when running the service management subcommands:

```console
$ sudo cloudflared service install ...
service installation is not supported on this operating system
```

```console
$ sudo cloudflared service uninstall ...
service uninstallation is not supported on this operating system
```

This keeps the available commands consistent until proper service
management support can be added for these otherwise-supported operating
systems.

cmd/cloudflared/generic_service.go

@@ -3,11 +3,36 @@
 package main
 
 import (
+	"fmt"
 	"os"
 
 	cli "github.com/urfave/cli/v2"
 )
 
 func runApp(app *cli.App, graceShutdownC chan struct{}) {
+	app.Commands = append(app.Commands, &cli.Command{
+		Name:  "service",
+		Usage: "Manages the cloudflared system service (not supported on this operating system)",
+		Subcommands: []*cli.Command{
+			{
+				Name:   "install",
+				Usage:  "Install cloudflared as a system service (not supported on this operating system)",
+				Action: cliutil.ConfiguredAction(installGenericService),
+			},
+			{
+				Name:   "uninstall",
+				Usage:  "Uninstall the cloudflared service (not supported on this operating system)",
+				Action: cliutil.ConfiguredAction(uninstallGenericService),
+			},
+		},
+	})
 	app.Run(os.Args)
 }
+
+func installGenericService(c *cli.Context) error {
+	return fmt.Errorf("service installation is not supported on this operating system")
+}
+
+func uninstallGenericService(c *cli.Context) error {
+	return fmt.Errorf("service uninstallation is not supported on this operating system")
+}