back-port shutdown assert fix

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9881 a83610d8-ad2a-0410-a6ab-fc0612d85776
merged proxy fix for adding .torrent file by URL from RC_0_16

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9883 a83610d8-ad2a-0410-a6ab-fc0612d85776
regenerated html

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9885 a83610d8-ad2a-0410-a6ab-fc0612d85776