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.

HISTORY

@@ -12,6 +12,7 @@
           provided to "node check" (Ian)
         repmgr: improve "node rejoin" checks (Ian)
         repmgr: enable "node rejoin" to join a target with a lower timeline (Ian)
+        repmgr: support pg_rewind's automatic crash recovery in Pg13 and later (Ian)
         repmgr: improve output formatting for cluster matrix/crosscheck (Ian)
         repmgr: improve database connection failure error checking on the
           demotion candidate during "standby switchover" (Ian)