mu: default to include related, skip dups

Add optioins --include-dups and --skip-related that are the reverse of
the previous ones. Leave the old options (hidden) for backward
compat (ie., scripts that use those options)

mu/mu-config.h

@@ -140,12 +140,11 @@ struct _MuConfig {
 	gchar		*exec;		/* command to execute on the
 					 * files for the matched
 					 * messages */
-	gboolean        skip_dups;        /* if there are multiple
-					 * messages with the same
-					 * msgid, show only the first
-					 * one */
-	gboolean        include_related; /* included related messages
-					  * in results */
+	gboolean        include_dups;    /* if there are multiple messages with
+					  * the same msgid, show all of them,
+					  * not just the first */
+	gboolean        skip_related;      /* do not include related messages in
+					  * results */
 	/* for find and cind */
 	time_t            after;          /* only show messages or
 					   * addresses last seen after