Add -E flag when attaching or switching client to bypass

update-environment, from Steven Lu.

tmux.h

@@ -1801,7 +1801,7 @@ extern const struct cmd_entry cmd_wait_for_entry;
 
 /* cmd-attach-session.c */
 enum cmd_retval	 cmd_attach_session(struct cmd_q *, const char *, int, int,
-		     const char *);
+		     const char *, int);
 
 /* cmd-list.c */
 struct cmd_list	*cmd_list_parse(int, char **, const char *, u_int, char **);