repmgr: add -q/--quiet option

This suppresses log output below log level ERROR. This is useful mainly
when repmgr is being executed programmatically, e.g. in a cronjob,
where it's only useful to receive output if something goes wrong.

Note we advise against using this option when executing repmgr
commands which operate on PostgreSQL nodes (standby follow,
standby promote, standby switchover, node rejoin), particularly when
executed by repmgrd, as the log output will provide valuable
troubleshooting information.

Implements suggestion in GitHub #468.

log.h

@@ -129,6 +129,7 @@ bool		logger_shutdown(void);
 void		logger_set_verbose(void);
 void		logger_set_terse(void);
 void		logger_set_min_level(int min_log_level);
+void		logger_set_level(int new_log_level);
 
 void
 log_detail(const char *fmt,...)