Startify unfortunately does not set the buftype option, so the
`[noperm]` would be shown, since the buffer is not really a file.
(see mhinz/vim-startify#324)
So allow to blacklist startify from this setting. To do so, the logic
from the tabline extension was refactored out into a separate function
that returns true, if the buffer matches
`g:airline#extensions#tabline#ignore_bufadd_pat` and that function is
then reused for the readonly() function as well.
b84e13f1d7