enable TCP keepalive for socks5 connection for UDP associate

git-svn-id: http://libtorrent.googlecode.com/svn@10325 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
merged socks5 keep-alive fix from RC_0_16

git-svn-id: http://libtorrent.googlecode.com/svn@10326 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda
update to head of RC_0_16

git-svn-id: http://libtorrent.googlecode.com/svn@10327 f43f7eb3-cfe1-5f9d-1b5f-e45aa6702bda