Handle discard all logfiles properly

Fixes #6892.

The [docs](https://www.consul.io/docs/agent/options.html#_log_rotate_max_files) are stating:

> -log-rotate-max-files - to specify the maximum number of older log
> file archives to keep. Defaults to 0 (no files are ever deleted). Set to
> -1 to disable rotation and discard all log files.

But the `-1` case was not implemented and led to a panic when being
used.

logger/logfile.go

@@ -104,8 +104,14 @@ func (l *LogFile) pruneFiles() error {
 	if err != nil {
 		return err
 	}
-	// Prune if there are more files stored than the configured max
-	stale := len(matches) - l.MaxFiles
+	var stale int
+	if l.MaxFiles == -1 {
+		// Prune everything
+		stale = len(matches)
+	} else {
+		// Prune if there are more files stored than the configured max
+		stale = len(matches) - l.MaxFiles
+	}
 	for i := 0; i < stale; i++ {
 		if err := os.Remove(matches[i]); err != nil {
 			return err

logger/logfile_test.go

@@ -136,7 +136,7 @@ func TestLogFile_deleteArchives(t *testing.T) {
 
 func TestLogFile_deleteArchivesDisabled(t *testing.T) {
 	t.Parallel()
-	tempDir := testutil.TempDir(t, "LogWriteDeleteArchivesDisabled")
+	tempDir := testutil.TempDir(t, t.Name())
 	defer os.Remove(tempDir)
 	filt := LevelFilter()
 	filt.MinLevel = logutils.LogLevel("INFO")
@@ -158,3 +158,28 @@ func TestLogFile_deleteArchivesDisabled(t *testing.T) {
 		return
 	}
 }
+
+func TestLogFile_rotationDisabled(t *testing.T) {
+	t.Parallel()
+	tempDir := testutil.TempDir(t, t.Name())
+	defer os.Remove(tempDir)
+	filt := LevelFilter()
+	filt.MinLevel = logutils.LogLevel("INFO")
+	logFile := LogFile{
+		logFilter: filt,
+		fileName:  testFileName,
+		logPath:   tempDir,
+		MaxBytes:  testBytes,
+		duration:  24 * time.Hour,
+		MaxFiles:  -1,
+	}
+	logFile.Write([]byte("[INFO] Hello World"))
+	logFile.Write([]byte("[INFO] Second File"))
+	logFile.Write([]byte("[INFO] Third File"))
+	want := 1
+	tempFiles, _ := ioutil.ReadDir(tempDir)
+	if got := tempFiles; len(got) != want {
+		t.Errorf("Expected %d files, got %v file(s)", want, len(got))
+		return
+	}
+}