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.
from the parent (global) tree, instead make it remove by name like options.
While here, also tidy up a few bits of options and hooks handling (use
RB_FOREACH_SAFE, and a helper function for the free).
