[RFC] input/job: process ctrl+c and do conversion in the read callback

- Extract `process_interrupts` out of `convert_input`
- Instead of waiting for os_breakcheck/os_inchar calls, call `convert_input`
  and `process_interrupts` directly from the read callback in input.c.
- Remove the `settmode` calls from `job_wait`. Now that interrupts are
  processed in the event loop, there's no need to set the terminal to cooked
  which introduces other problems(ref [7.4.427](https://github.com/neovim/neovim/pull/1336))

This is also good because it will simplify handling ctrl+c in the middle of server->client msgpack-rpc calls