More tweaks to the HTML related sniffing rules:

* MHTML: give a higher priority than HTML to improve detection results,
as MHTL is likely to contain HTML in the first chunk.
* HTML: scan the first 512 bytes to increase likeliness of finding a tag
* XHTML: don't try to identify on the dectype, this is not reliable and
missing from some documents. Recognize "<html xmlns" and the utf-16
equivalent, as well as part of the DTD tag.
Remove some traces sent to application stdout.

* This may be fine in media server where it lands in logs, but certainly
not when the code is run in applications.
* Helps with all media-related webkit tests.