Relax <select> parser

This patch makes the parser allow additional tags in <select> besides
<option>, <optgroup>, and <hr>, mostly by removing the "in select" and
"in select in table" parser modes.

In order to replicate the behavior where opening a <select> tag within
another open <select> tag inserts a </select> close tag, a traversal
through the stack of open elements was added which I borrowed from the
<button> part of the parser.

This will need test changes to be implemented in html5lib.

Fixes #10310