repmgr standby switchover: Improve handling of node rejoin failure on the demotion candidate.

Previously repmgr did not check whether repmgr node rejoin actually succeeded on the demotion candidate, and would always wait up to node_rejoin_timeout seconds for it to attach to the promotion candidate, even if this would never happen.

This makes it easier to identify unexpected events during a switchover operation, such as the demotion candidate being unexpectedly restarted by an external process.

Note that the output of the repmgr node rejoin operation on the demotion candidate will now be logged to a temporary file on that node; the location of the file will be reported in the error message, if one is emitted.

repmgrd: at startup, if node record is marked as "inactive", attempt to set it to "active".

This behaviour can be overridden by setting the configuration parameter repmgrd_exit_on_inactive_node to true.

repmgr node rejoin: emit rejoin target note information as NOTICE.

This makes it clearer what repmgr is trying to do.

repmgr node check: option --repmgrd added to check repmgrd status.

Add %p event notification parameter providing the node ID of the former primary for the repmgrd_failover_promote event.

repmgr standby clone: if using --replication-conf-only on a node which was set up without replication slots, but the repmgr configuration was since changed to use_replication_slots=1, repmgr will now set slot_name in the node record, if it was previously empty.

repmgrd: rename internal shared library functions to minimize the risk of clashes with other shared libraries.

This does not affect user-facing SQL functions. However an upgrade of the installed extension version is required.

repmgrd: ensure short option -s is accepted.