Add support for custom timestamps in chat and logs.

Internal representations of the time_format setting are replaced by timestamp format strings.
time_format is only used in the setup.

Signed-off-by: Loui Chang <louipc.ist@gmail.com>

src/misc_tools.c

@@ -87,7 +87,7 @@ void get_time_str(char *buf, int bufsize)
         return;
     }
 
-    const char *t = user_settings->time == TIME_12 ? "%I:%M:%S " : "%H:%M:%S ";
+    const char *t = user_settings->timestamp_format;
     strftime(buf, bufsize, t, get_time());
 }