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.

tmux.h

@@ -1339,8 +1339,7 @@ struct cmd_q {
 	int			 references;
 	int			 flags;
 #define CMD_Q_DEAD 0x1
-#define CMD_Q_REENTRY 0x2
-#define CMD_Q_NOHOOKS 0x4
+#define CMD_Q_NOHOOKS 0x2
 
 	struct client		*client;
 	int			 client_exit;
@@ -1404,6 +1403,7 @@ struct cmd_entry {
 
 #define CMD_STARTSERVER 0x1
 #define CMD_READONLY 0x2
+#define CMD_AFTERHOOK 0x4
 	int		 flags;
 
 	enum cmd_retval		 (*exec)(struct cmd *, struct cmd_q *);