There aren't many client message types or code to handle them so get rid of
the lookup table and use a switch, merge the tiny handler functions into it,
and move the whole lot to client.c.

Also change client_msg_dispatch to consume as many messages as possible and
move the call to it to the right place so it checks for signals afterwards.

Prompted by suggestions from eric@.

Rename struct hdrtype to msgtype which is a better name and can be used even
when struct hdr disappears.

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.

Tidy client message return slightly: convert flags into an enum, and merge
error string into struct client_ctx as well.

Nix a few unused attributes on arguments which are no longer unused.

properly and print a nicer message. Same effect though :-)