Rename "logxxx" configuration file parameters to "log_xxx"

This is more consistent with other parameters and conforms to
the pattern used by PostgreSQL itself, which uses the prefix "log_"
for logging parameters.

A warning will be emitted if the old version of the parameter name
is detected.

doc/changes-in-repmgr4.md

@@ -28,6 +28,7 @@ Changed command line options
   configuration file option `monitoring_history`. This enables the
   setting to be changed without having to modify system service files.
 
+
 Removed configuration file options
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