Extend the modifiers allowed before formats: as well as the existing

#{=10:...}  length limit, add #{t:...} to convert a time_t format to a
string, #{b:...} for basename and #{d:...} for dirname. Remove all the
foo_string time formats as they can now be replaced by "t:", for example
#{window_activity_string} becomes #{t:window_activity}.

cmd-choose-client.c

@@ -31,7 +31,7 @@
 	"#{client_tty}: #{session_name} "			\
 	"[#{client_width}x#{client_height} #{client_termname}]"	\
 	"#{?client_utf8, (utf8),}#{?client_readonly, (ro),} "	\
-	"(last used #{client_activity_string})"
+	"(last used #{t:client_activity})"
 
 enum cmd_retval	 cmd_choose_client_exec(struct cmd *, struct cmd_q *);