fs_open: return EROFS for non-RO opens

The use of EACCES leads to slightly confusing error messages in
downstream consumers, so prefer EROFS to better articulate what's
actually happening.

While here, use O_RDWR to mask the open flags while testing for
non-RO access.  This is at least encouraged by POSIX with their
suggestion that "O_RDONLY | O_WRONLY == O_RDWR".

src/fuse_local.c

@@ -95,8 +95,8 @@ static int fs_open(const char *path, struct fuse_file_info *fi)
     if (!link) {
         return -ENOENT;
     }
-    if ((fi->flags & 3) != O_RDONLY) {
-        return -EACCES;
+    if ((fi->flags & O_RDWR) != O_RDONLY) {
+        return -EROFS;
     }
     if (CACHE_SYSTEM_INIT) {
         fi->fh = (uint64_t) Cache_open(path);