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/ex_cmds2.c

@@ -2372,9 +2372,9 @@ int source_level(void *cookie)
  */
 static FILE *fopen_noinh_readbin(char *filename)
 {
-  int fd_tmp = mch_open(filename, O_RDONLY, 0);
+  int fd_tmp = os_open(filename, O_RDONLY, 0);
 
-  if (fd_tmp == -1)
+  if (fd_tmp < 0)
     return NULL;
 
 # ifdef HAVE_FD_CLOEXEC