bug#43217: EWW ignores select options in an optgroup

[Nicolas Graner <nicolas.graner@universite-paris-saclay.fr>]

When rendering <select> tags, EWW ignores any <option> which is part
of an <optgroup>.

Example : put this into an HTML file and visit it in EWW:

--------------------------------------------------
<form method="post">
first select
<select name="a">
<option value="1">one</option>
<option value="2">two</option>
</select>
second select
<select name="b">
<optgroup label="foobar">
<option value="3">three</option>
<option value="4">four</option>
</optgroup>
</select>
</form>
--------------------------------------------------

It should normally display:

--------------------------------------------------
first select 

one

second select 

three
--------------------------------------------------

but instead displays:

--------------------------------------------------
first select 

one

second select 
--------------------------------------------------

Explanation: the function 'eww-tag-select' searches for 'option' in
(dom-non-text-children dom) which includes only first-level children
of the 'select' node, and therefore misses children of 'optgroup' children.

Note: this was tested in master. Earlier versions had the opposite bug
where options where processed *only* if they were within an optgroup.
It seems that this bug was fixed incorrectly.

Nicolas