refactor: delegate -l output to findmnt

The custom one-alias-per-line output was useless — no mountpoint, no
source, no options. Reinventing a table format when findmnt from
util-linux already produces a familiar fuse.sshfs view was the wrong
call. -l now shells out to findmnt -t fuse.sshfs.

main_test.go

@@ -4,7 +4,6 @@
 package main
 
 import (
-	"bytes"
 	"os"
 	"path/filepath"
 	"strings"
@@ -140,36 +139,6 @@ func TestVerifyMountDirRejectsSymlink(t *testing.T) {
 	}
 }
 
-func TestListMountsMissingBase(t *testing.T) {
-	runtime := t.TempDir()
-	t.Setenv("XDG_RUNTIME_DIR", runtime)
-	var buf bytes.Buffer
-	if err := list_mounts(&buf); err != nil {
-		t.Fatalf("unexpected error: %v", err)
-	}
-	if buf.Len() != 0 {
-		t.Fatalf("want empty output, got %q", buf.String())
-	}
-}
-
-func TestListMountsFiltersUnmounted(t *testing.T) {
-	runtime := t.TempDir()
-	t.Setenv("XDG_RUNTIME_DIR", runtime)
-	base := filepath.Join(runtime, "sshfs")
-	for _, name := range []string{"stale1", "stale2"} {
-		if err := os.MkdirAll(filepath.Join(base, name), 0700); err != nil {
-			t.Fatalf("setup stale dir: %v", err)
-		}
-	}
-	var buf bytes.Buffer
-	if err := list_mounts(&buf); err != nil {
-		t.Fatalf("unexpected error: %v", err)
-	}
-	if buf.Len() != 0 {
-		t.Fatalf("want stale dirs filtered, got %q", buf.String())
-	}
-}
-
 func TestUnmountRejectsBadAlias(t *testing.T) {
 	runtime := t.TempDir()
 	t.Setenv("XDG_RUNTIME_DIR", runtime)