*w3m form textarea* buffer, not modified, readonly

[jidanni@xxxxxxxxxxx]

Regarding the *w3m form textarea* buffer:
If the user hasn't started changing anything in it, it should be
marked "--" unmodified, as seen in the minibuffer, and Buffer List,
not "**" modified.

Furthermore, if called via <textarea readonly="readonly">, then it
should also be "%%" readonly, like how firefox acts too.