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.

repmgr-client-global.h

@@ -32,7 +32,7 @@ typedef struct
 	bool		wait;
 
 	/* logging options */
-	char		loglevel[MAXLEN];  /* overrides setting in repmgr.conf */
+	char		log_level[MAXLEN];  /* overrides setting in repmgr.conf */
 	bool		log_to_file;
 	bool		terse;
 	bool		verbose;