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.test.node2.conf

@@ -5,4 +5,4 @@ upstream_node_id=1
 node_name='node2'
 use_replication_slots = true
 conninfo = 'host=127.0.0.1 dbname=repmgr user=repmgr port=5501 connect_timeout=2'
-loglevel = 'DEBUG'
+log_level = 'DEBUG'