sdjournal: add Rewind method to JournalReader

This allows users of the JournalReader to rewind to the beginning of the
Journal.

Also add a missing docstring to JournalReader.Close

sdjournal/read.go

@@ -125,10 +125,16 @@ func (r *JournalReader) Read(b []byte) (int, error) {
 	return len(msg), nil
 }
 
+// Close closes the JournalReader's handle to the journal.
 func (r *JournalReader) Close() error {
 	return r.journal.Close()
 }
 
+// Rewind attempts to rewind the JournalReader to the first entry.
+func (r *JournalReader) Rewind() error {
+	return r.journal.SeekHead()
+}
+
 // Follow synchronously follows the JournalReader, writing each new journal entry to writer. The
 // follow will continue until a single time.Time is received on the until channel.
 func (r *JournalReader) Follow(until <-chan time.Time, writer io.Writer) (err error) {