open_buffer(): Do BufEnter for directories.

Abuse NOTDONE to give some nuance to the return value of readfile(), so
that open_buffer() can distinguish between "failed, lol" and "failed
because the path is a directory".

Before this change, Vim *already* creates a new buffer when a directory
is edited. So there is no reason it should not raise BufEnter, that was
an implementation detail of ye olde readfile().

Most of the changes in this commit merely preserve the old semantics.
The "implicit" change that we actually are interested in, is this line
in `open_buffer()`, where `retval` being non-FAIL allows EVENT_BUFENTER
to be applied:

    apply_autocmds_retval(EVENT_BUFENTER, NULL, NULL, FALSE, curbuf, &retval);

References https://github.com/vim/vim/issues/1353

src/nvim/buffer.c

@@ -193,10 +193,13 @@ open_buffer (
        * it can be changed there. */
       if (!readonlymode && !bufempty())
         changed();
-      else if (retval != FAIL)
+      else if (retval == OK) {
         unchanged(curbuf, FALSE);
-      apply_autocmds_retval(EVENT_STDINREADPOST, NULL, NULL, FALSE,
-          curbuf, &retval);
+      }
+      if (retval == OK) {
+        apply_autocmds_retval(EVENT_STDINREADPOST, NULL, NULL, FALSE,
+                              curbuf, &retval);
+      }
     }
   }
 
@@ -219,7 +222,7 @@ open_buffer (
       || (aborting() && vim_strchr(p_cpo, CPO_INTMOD) != NULL)
       )
     changed();
-  else if (retval != FAIL && !read_stdin)
+  else if (retval == OK && !read_stdin)
     unchanged(curbuf, FALSE);
   save_file_ff(curbuf);                 /* keep this fileformat */