Client tidying: get rid of client_ctx struct in favour of two variables in
client.c, and move the functions in client-fn.c into other files.

Switch tmux to use imsg. This is the last major change to make the
client-server protocol more resilient and make the protocol versioning work
properly. In future, the only things requiring a protocol version bump will be
changes in the message structs, and (when both client and server have this
change) mixing different versions should nicely report an error message.

As a side effect this also makes the code tidier, fixes a problem with the way
errors reported during server startup were handled, and supports fd passing
(which will be used in future).

Looked over by eric@, thanks.

Please note that mixing a client with this change with an older server or vice
versa may cause tmux to crash or hang - tmux should be completely exited before
upgrading.

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.