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

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

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.

Use KEYC_NONE constant instead of 0 on init.

Give each paste buffer a size member instead of requiring them to be
zero-terminated.

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.

maintain and is only going to get worse as more are used. So instead, add a new
uint64_t member to cmd_entry which is a bitmask of upper and lowercase options
accepted by the command.

This means new single character options can be used without the need to add it
explicitly to the list.