Remove unused stylesheet from tail.html.  Closes #521
Fix shutdown error in close_httpservers()
"RuntimeError: dictionary changed size during iteration" would sometimes
be thrown.  A way to make it happen is to load a page on the web interface.
This was caused by 88dc3edd2d487742def6f3cc3ab63cc85a99e423, before that
change close_httpservers() wasn't called during shutdown.