html: process </td> tags; foster parent at most one node per token
Correctly close table cell when </td> is read.
Because of reconstructing the active formatting elements, more than one
node may be created when reading a single token.
If both nodes are foster parented, they will be siblings, but the first
node should be the parent of the second.
Pass tests1.dat, test 77:
<a href="blah">aba<table><a href="foo">br<tr><td></td></tr>x</table>aoe
| <html>
| <head>
| <body>
| <a>
| href="blah"
| "aba"
| <a>
| href="foo"
| "br"
| <a>
| href="foo"
| "x"
| <table>
| <tbody>
| <tr>
| <td>
| <a>
| href="foo"
| "aoe"
R=nigeltao
CC=golang-dev
https://golang.org/cl/
5305074