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.1

@@ -3204,20 +3204,16 @@ shows only the option value, not the name.
 .Nm
 allows commands to run on various triggers, called
 .Em hooks .
-Each
+Most
 .Nm
-command has a
-.Em before
-hook and an
+commands have an
 .Em after
 hook and there are a number of hooks not associated with commands.
 .Pp
-A command's before hook is run before the command is executed and its after
-hook is run afterwards, except when the command is run as part of a hook
+A command's after
+hook is run after it completes, except when the command is run as part of a hook
 itself.
-Before hooks are named using the
-.Ql before-
-prefix and after hooks the
+They are named with an
 .Ql after-
 prefix.
 For example, the following command adds a hook to select the even-vertical
@@ -3227,11 +3223,6 @@ layout after every
 set-hook after-split-window "selectl even-vertical"
 .Ed
 .Pp
-Or to write when each new window is created to a file:
-.Bd -literal -offset indent
-set-hook before-new-window 'run "date >>/tmp/log"'
-.Ed
-.Pp
 In addition, the following hooks are available:
 .Bl -tag -width "XXXXXXXXXXXXXXXX"
 .It alert-activity