Trying to do hooks generically is way too complicated and unreliable and

confusing, particularly trying to automatically figure out what target
hooks should be using. So simplify it:

- drop before hooks entirely, they don't seem to be very useful;

- commands with special requirements now fire their own after hook (for
  example, if they change session or window, or if they have -t and -s
  and need to choose which one the hook uses as current target);

- commands with no special requirements can have the CMD_AFTERHOOK flag
  added and they will use the -t state.

At the moment new-session, new-window, split-window fire their own hook,
and display-message uses the flag. The remaining commands still need to
be looked at.

cmd-display-message.c

@@ -45,7 +45,7 @@ const struct cmd_entry cmd_display_message_entry = {
 	.cflag = CMD_CLIENT_CANFAIL,
 	.tflag = CMD_PANE,
 
-	.flags = 0,
+	.flags = CMD_AFTERHOOK,
 	.exec = cmd_display_message_exec
 };