repmgr: add --terse mode to "cluster show"

This suppresses display of the usually lengthy "conninfo" column, mainly
useful for generating a compact table suitable for pasting into emails,
chats etc. without messy line breaks.

Implements GitHub #521.

repmgr-client-global.h

@@ -199,6 +199,7 @@ typedef struct ColHeader
 	char		title[MAXLEN];
 	int			max_length;
 	int			cur_length;
+	bool		display;
 } ColHeader;