node rejoin: handle unclean shutdown in Pg13

From PostgreSQL 13, pg_rewind will automatically handle an unclean
shutdown itself, so as long as --force-rewind was provided, so there
is no need to fail with an error.

Note that pg_rewind handles the unclean shutdown by starting PostgreSQL
in single user mode, which it does before performing any checks as
to whether a rewind is actually necessary.

However pg_rewind doesn't take into account the possible presence
of a standby.signal file, so we remove that and recreate it after
pg_rewind was executed.

doc/appendix-release-notes.xml

@@ -115,6 +115,15 @@
             </para>
           </listitem>
 
+          <listitem>
+            <para>
+              <link linkend="repmgr-node-rejoin">repmgr node rejoin</link>:
+              in PostgreSQL 13 and later, support <application>pg_rewind</application>'s
+              ability to automatically run crash recovery on a PostgreSQL instance
+              which was not shut down cleanly.
+            </para>
+          </listitem>
+
           <listitem>
             <para>
               <link linkend="repmgr-node-check">repmgr node check</link>: