cmd: Split unix sockets for admin endpoint addresses (#5696)

* cmd: fix cli when admin endpoint uses new unix socket permission format

Fixes a bug where the following Caddyfile

```Caddyfile
{
	admin unix/admin.sock|0660
}
```

and `caddy reload --config Caddyfile`
would throw the following error instead of reloading it:

```
INFO    using provided configuration    {"config_file": "Caddyfile", "config_adapter": ""}
Error: sending configuration to instance: performing request: Post "http://127.0.0.1/load": dial unix admin.sock|0660: connect: no such file or directory
[ERROR] exit status 1
```

---

This bug also affected `caddy start` and `caddy stop`.

* Move splitter function to internal

---------

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>

listeners_test.go

@@ -17,6 +17,8 @@ package caddy
 import (
 	"reflect"
 	"testing"
+
+	"github.com/caddyserver/caddy/v2/internal"
 )
 
 func TestSplitNetworkAddress(t *testing.T) {
@@ -634,7 +636,7 @@ func TestSplitUnixSocketPermissionsBits(t *testing.T) {
 			expectErr: true,
 		},
 	} {
-		actualPath, actualFileMode, err := splitUnixSocketPermissionsBits(tc.input)
+		actualPath, actualFileMode, err := internal.SplitUnixSocketPermissionsBits(tc.input)
 		if tc.expectErr && err == nil {
 			t.Errorf("Test %d: Expected error but got: %v", i, err)
 		}