fs.c: implement os_file_is_readable()

Use access() because:

- We already use it for os_file_is_writable()
- Vim's old check_file_readonly() ends up using access() after all.

src/nvim/eval.c

@@ -8562,27 +8562,12 @@ static void f_feedkeys(typval_T *argvars, typval_T *rettv)
   }
 }
 
-/*
- * "filereadable()" function
- */
+/// "filereadable()" function
 static void f_filereadable(typval_T *argvars, typval_T *rettv)
 {
-  int fd;
-  char_u      *p;
-  int n;
-
-#ifndef O_NONBLOCK
-# define O_NONBLOCK 0
-#endif
-  p = get_tv_string(&argvars[0]);
-  if (*p && !os_isdir(p) && (fd = os_open((char *)p,
-                                  O_RDONLY | O_NONBLOCK, 0)) >= 0) {
-    n = TRUE;
-    close(fd);
-  } else
-    n = FALSE;
-
-  rettv->vval.v_number = n;
+  char_u *p = get_tv_string(&argvars[0]);
+  rettv->vval.v_number =
+    (*p && !os_isdir(p) && os_file_is_readable((char*)p));
 }
 
 /*

src/nvim/os/fs.c

@@ -279,13 +279,22 @@ bool os_file_is_readonly(const char *name)
   return access(name, W_OK) != 0;
 }
 
+/// Check if a file is readable.
+///
+/// @return true if `name` is readable, otherwise false.
+bool os_file_is_readable(const char *name)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  return (access(name, R_OK) == 0);
+}
+
 /// Check if a file is writable.
 ///
 /// @return `0` if `name` is not writable,
 /// @return `1` if `name` is writable,
 /// @return `2` for a directory which we have rights to write into.
 int os_file_is_writable(const char *name)
-  FUNC_ATTR_NONNULL_ALL
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
   if (access(name, W_OK) == 0) {
     if (os_isdir((char_u *)name)) {