MAJOR: http: update connection mode configuration

At the very beginning of haproxy, there was "option httpclose" to make
haproxy add a "Connection: close" header in both directions to invite
both sides to agree on closing the connection. It did not work with some
rare products, so "option forceclose" was added to do the same and actively
close the connection. Then client-side keep-alive was supported, so option
http-server-close was introduced. Now we have keep-alive with a fourth
option, not to mention the implicit tunnel mode.

The connection configuration has become a total mess because all the
options above may be combined together, despite almost everyone thinking
they cancel each other, as judging from the common problem reports on the
mailing list. Unfortunately, re-reading the doc shows that it's not clear
at all that options may be combined, and the opposite seems more obvious
since they're compared. The most common issue is options being set in the
defaults section that are not negated in other sections, but are just
combined when the user expects them to be overloaded. The migration to
keep-alive by default will only make things worse.

So let's start to address the first problem. A transaction can only work in
5 modes today :
  - tunnel : haproxy doesn't bother with what follows the first req/resp
  - passive close : option http-close
  - forced close : option forceclose
  - server close : option http-server-close with keep-alive on the client side
  - keep-alive   : option http-keep-alive, end to end

All 16 combination for each section fall into one of these cases. Same for
the 256 combinations resulting from frontend+backend different modes.

With this patch, we're doing something slightly different, which will not
change anything for users with valid configs, and will only change the
behaviour for users with unsafe configs. The principle is that these options
may not combined anymore, and that the latest one always overrides all the
other ones, including those inherited from the defaults section. The "no
option xxx" statement is still supported to cancel one option and fall back
to the default one. It is mainly needed to ignore defaults sections (eg:
force the tunnel mode). The frontend+backend combinations have not changed.

So for examplen the following configuration used to put the connection
into forceclose :

    defaults http
        mode http
        option httpclose

    frontend foo.
        option http-server-close

  => http-server-close+httpclose = forceclose before this patch! Now
     the frontend's config replaces the defaults config and results in
     the more expected http-server-close.

All 25 combinations of the 5 modes in (frontend,backend) have been
successfully tested.

In order to prepare for upcoming changes, a new "option http-tunnel" was
added. It currently only voids all other options, and has the lowest
precedence when mixed with another option in another frontend/backend.
MEDIUM: http: make keep-alive + httpclose be passive mode

There's no particular reason for having keep-alive + httpclose combine
into forceclose when set in different frontend/backend sections, since
keep-alive does not close anything by default. Let's have this still
combination remain httpclose only.
MAJOR: http: switch to keep-alive mode by default

Since we support HTTP keep-alive, there is no more reason for staying
in tunnel mode by default. It is confusing for new users and creates
more issues than it solves. Option "http-tunnel" is available to force
to use it if really desired.

Switching to KA by default has implied to change the value of some
option flags and some transaction flags so that value zero (default)
matches keep-alive. That explains why more code has been changed than
expected. Tests have been run on the 25 combinations of frontend and
backend options, plus a few with option http-pretend-keepalive, and
no anomaly was found.

The relation between frontend and backends remains the same. Options
have been updated to take precedence over http-keep-alive which is now
implicit.

All references in the doc to haproxy not supporting keep-alive have
been fixed, and the doc for config options has been updated.