Invalid HTML due to form id

The HTML output for the masquerade block includes the following:

<form action="/page/url"  method="post" id="-masquerade-block-1-form">

In XHTML, the id attribute may not start with a "-". The forms API bases the form id off of the function used to build the form, _masquerade_block_1. You can add an #id value to the form to fix this problem, as I have in the attached patch.