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/eval.c

@@ -10675,6 +10675,7 @@ static void f_jobstart(typval_T *argvars, typval_T *rettv)
 
   job_start(argv,
             xstrdup((char *)argvars[0].vval.v_string),
+            true,
             on_job_stdout,
             on_job_stderr,
             on_job_exit,