Also add the EXITFREE definition to main_lib rather than the nvim target, as the header generation needs the EXITFREE flag to work properly.

Rename stdin/stdout in the server, so that RPC data won't get corrupted.
This also restores the use of stderr to write directly to the terminal.

feat(ui): refactor TUI from thread to separate process

As neovim does have event handling, we are checking for CTRL-C
all the time, not once per second.

Also, do_sleep() reimplements the same loop as
LOOP_PROCESS_EVENTS_UNTIL() already contains internally. Fix the latter
to use the right integer type, so we do not need the extra indirection.

Allow Include What You Use to remove unnecessary includes and only
include what is necessary. This helps with reducing compilation times
and makes it easier to visualise which dependencies are actually
required.

Work on https://github.com/neovim/neovim/issues/549, but doesn't close
it since this only works fully for .c files and not headers.

It's confusing to mix vendored dependencies with neovim source code. A
clean separation is simpler to keep track of and simpler to document.

Some V512 warnings have changed to V1086, and PVS apparently does not
know `uv_run()` can change `*timeout_expired`.