Fix default "log_directory" value

Although "log_directory" parameter's default value is changed in
PostgreSQL version 10, pg_rman didn't change it.

It leads following main issues if the "log_directory" is not specified.

1. Server logs are always backuped even if "--with-serverlog" is not
specified because pg_rman fails to exclude the "PGDATA/log" directory
from backup target lists.

2. pg_rman's "--keep-srvlog-files" and "--keep-srvlog-days" options are
invalid because SRVLOG_PATH is wrong.

init.c

@@ -176,9 +176,9 @@ do_init(void)
 		}
 		else if (pgdata)
 		{
-			/* default: log_directory = 'pg_log' */
+			/* default: log_directory = 'log' if PostgreSQL version is 10 or above */
 			srvlog_path = pgut_malloc(MAXPGPATH);
-			join_path_components(srvlog_path, pgdata, "pg_log");
+			join_path_components(srvlog_path, pgdata, "log");
 		}
 	}
 	if (srvlog_path)