BUG/MEDIUM: backend: do not re-initialize the connection's context upon reuse

If we reuse a server-side connection, we must not reinitialize its context nor
try to enable send_proxy. At the moment HTTP keep-alive over SSL fails on the
first attempt because the SSL context was cleared, so it only worked after a
retry.
BUG: Revert "OPTIM/MEDIUM: epoll: fuse active events into polled ones during polling changes"

This reverts commit 2f877304ef180654d165bf4ba8c88c204fc09d36.

This commit is OK for clear text traffic but causes trouble with SSL
when buffers are smaller than SSL buffers. Since the issue it addresses
will be gone once the polling redesign is complete, there's no reason
for trying to workaround temporary inefficiencies. Better remove it.
BUG/MINOR: checks: successful check completion must not re-enable MAINT servers

If a server is switched to maintenance mode while a check is in progress,
the successful completion of the check must not switch it back up. This
is still a consequence of using the same function set_server_up() for
every state change. Bug reported by Igor at owind.

This fix should be backported to 1.4 which is affected as well.
MINOR: http: try to stick to same server after status 401/407

In HTTP keep-alive mode, if we receive a 401, we still have a chance
of being able to send the visitor again to the same server over the
same connection. This is required by some broken protocols such as
NTLM, and anyway whenever there is an opportunity for sending the
challenge to the proper place, it's better to do it (at least it
helps with debugging).
Merge branch 'master' of http://git.1wt.eu/git/haproxy