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.

As a consequence buffer-limit is now a server option.

Massive spaces->tabs and trailing whitespace cleanup, hopefully for the last
time now I've configured emacs to make them displayed in really annoying
colours...

Remove a couple of unused arguments where possible, and add /* ARGSUSED */ to
the rest to reduce lint output.

Tweak a comment and add some spacing.

Get rid of the ugly CMD_CHFLAG macro and use a const string (eg "dDU") in the
command entry structs and a couple of functions to check/set the flags.

Initialise the arg2 pointer properly (also free it when freeing the
others). Fixes crashes with J in malloc_options reported by oga.

Add flags for 1+2 and 2 arguments to the generic target code, use it for
cmd-set-environment/option/window-option and remove the generic options
parsing.

Merge pane number into the target specification for pane commands. Instead of
using -p index, a target pane is now addressed with the normal -t window form
but suffixed with a period and a pane index, for example :0.2 or
mysess:mywin.1. An unadorned number such as -t 1 is tried as a pane index in
the current window, if that fails the same rules are followed as for a target
window and the current pane in that window used.

As a side-effect this now means that swap-pane can swap panes between
different windows.

Note that this changes the syntax of the break-pane, clear-history, kill-pane,
resize-pane, select-pane and swap-pane commands.