CMD_FIND_* flags in the cmd_entry and call it for the command. Commands
with special requirements call it themselves and update the target for
hooks to use.
the state (client, session, winlink, pane) for it it before entering the
command. Each command provides some flags that tell the prepare step
what it is expecting.
This is a requirement for having hooks on commands (for example, if you
hook "select-window -t1:2", the hook command should to operate on window
1:2 not whatever it thinks is the current window), and should allow some
other target improvements.
The old cmd_find_* functions remain for the moment but that layer will
be dropped later.
Joint work with Thomas Adam.
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).
