os_open: impl mch_open with libuv. ref #133

- use return value instead of open_req.result
- libuv uv_fs_open() returns `-errno` instead of always -1
- libuv always sets open_req.result to the return value, _except_ for OOM
  where it only sets the return value. So always use the return value.
- replace calls to mch_open macro.
- update call sites expecting -1 error

src/nvim/eval.c

@@ -8317,7 +8317,7 @@ static void f_filereadable(typval_T *argvars, typval_T *rettv)
 # define O_NONBLOCK 0
 #endif
   p = get_tv_string(&argvars[0]);
-  if (*p && !os_isdir(p) && (fd = mch_open((char *)p,
+  if (*p && !os_isdir(p) && (fd = os_open((char *)p,
                                   O_RDONLY | O_NONBLOCK, 0)) >= 0) {
     n = TRUE;
     close(fd);