Make command exec functions return an enum rather than -1/0/1 values and
add a new value to mean "leave client running but don't attach" to fix
problems with using some commands in a command sequence. Most of the
work by Thomas Adam, problem reported by "jspenguin" on SF bug 3535531.

Drop the ability to have a list of keys in the prefix in favour of two
separate options, prefix and prefix2. This simplifies the code and gets
rid the data options type which was only used for this one option.

Also add a -2 flag to send-prefix to send the secondary prefix key,
fixing a cause of minor irritation.

People who want three prefix keys are out of luck :-).

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.

We were only ever using the client to find the session anyway.

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.

Permit multiple prefix keys to be defined, separated by commas, for example:

set -g prefix ^a,^b

Any key in the list acts as the prefix. The send-prefix command always sends
the first key in the list.

Now that pane targets (-t) are supported, switch some commands to use them
where it makes sense: clock-mode, copy-mode, scroll-mode, send-keys,
send-prefix.

Make all messages sent between the client and server fixed size.

This is the first of two changes to make the protocol more resilient and less
sensitive to other changes in the code, particularly with commands. The client
now packs argv into a buffer and sends it to the server for parsing, rather
than doing it itself and sending the parsed command data.

As a side-effect this also removes a lot of now-unused command marshalling
code.

Mixing a server without this change and a client with or vice versa will cause
tmux to hang or crash, please ensure that tmux is entirely killed before
upgrading.