job: Let job_start callers to selectively ignore stdio

Passing NULL as the callback for stdout/stderr will result in job_start ignoring
stdout/stderr, respectively. A 'writable' boolean argument was also added, and
when false `job_start` will ignore stdin.

Also, refactor os_system to allow passing NULL as the `output` argument.

src/nvim/msgpack_rpc/channel.c

@@ -119,6 +119,7 @@ uint64_t channel_from_job(char **argv)
   int status;
   channel->data.job = job_start(argv,
                                 channel,
+                                true,
                                 job_out,
                                 job_err,
                                 job_exit,