pg_upgrade: pass username to analyze script

If -U (user) is specified, pass the username into the created analyze
script.
Per request from Ray Stell

contrib/pg_upgrade/pg_upgrade.h

@@ -291,6 +291,7 @@ typedef struct
 	const char *progname;		/* complete pathname for this program */
 	char	   *exec_path;		/* full path to my executable */
 	char	   *user;			/* username for clusters */
+	bool		user_specified; /* user specified on command-line */
 	char	  **old_tablespaces;	/* tablespaces */
 	int			num_old_tablespaces;
 	char	  **libraries;		/* loadable libraries */