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-list-sessions.c

@@ -30,7 +30,7 @@
 
 #define LIST_SESSIONS_TEMPLATE				\
 	"#{session_name}: #{session_windows} windows "	\
-	"(created #{session_created_string}) "		\
+	"(created #{t:session_created}) "		\
 	"[#{session_width}x#{session_height}]"		\
 	"#{?session_grouped, (group ,}"			\
 	"#{session_group}#{?session_grouped,),}"	\