cleanup: detect dmeventd_executable just once

Avoid repeating debug messages about dmeventd executable
and just remember it once for whole cmd lifetime.

lib/commands/toolcontext.h

@@ -42,6 +42,7 @@ struct config_info {
 	int cache_vgmetadata;
 	const char *msg_prefix;
 	const char *fmt_name;
+	const char *dmeventd_executable;
 	uint64_t unit_factor;
 	int cmd_name;		/* Show command name? */
 	mode_t umask;