Clean up and simplify tmux command argument parsing.
Originally, tmux commands were parsed in the client process into a
struct with the command data which was then serialised and sent to the
server to be executed. The parsing was later moved into the server (an
argv was sent from the client), but the parse step and intermediate
struct was kept.
This change removes that struct and the separate parse step. Argument
parsing and printing is now common to all commands (in arguments.c) with
each command left with just an optional check function (to validate the
arguments at parse time), the exec function and a function to set up any
key bindings (renamed from the old init function).
This is overall more simple and consistent.
There should be no changes to any commands behaviour or syntax although
as this touches every command please watch for any unexpected changes.
Add a choose-client command and extend choose-{session,window} to accept a
template. After a choice is made, %% (or %1) in the template is replaced by the
name of the session, window or client suitable for -t and the result executed
as a command. So, for example, "choose-window "killw -t '%%'"" will kill the
selected window.
The defaults if no template is given are (as now) select-window for
choose-window, switch-client for choose-session, and detach-client for
choose-client (now bound to D).
