Add filter/action to change form on profile page to enctype="multipart-form/data"

[pampfelimetten]

As already discused three years ago(#3890), it would be great to change the enctype to multipart to allow easier integration of file upload at the backend. There are some plugins which change it on the fly with javascript, but I think it would be a lot cleaner to just have a filter/action for it.

Also see discussion here: #10518

As wp moves on to a fully featured cms, it should support an easy way to add additional files associated with users.

Patch adds do_action to form tag of user_new.php edit_user.php and edit-form-advanced.php.

I have seen numerous requests to do this for the post edit form on IRC so figured I'd throw it in here too.

Adds action to add/edit user and to edit post pages.

I can't come up with a better way to do this, looks good.

We might as well add the same for terms and links, if we do this.

terms and links are a can of worms. let's get this in 3.0 and worry about the other ones in 3.1.

(In [14106]) Add actions to the form tags of user-new, user-edit, and post edit. props sivel, fixes #11933.