BUG/MEDIUM: http/auth: Sometimes the authentication credentials can be mix between two requests

The authentication function "get_http_auth()" extract credentials from
the request and keep it this values in shared cache. This function set
a flag in the session indicating that the authentication is already
parsed and the value stored in the cache are avalaible. If this flag is
set the authorization header is not re-parsed and the shared cache is
used.

If two request are simultaneous processsed, the first one check the
credentials. After this, the second request check also it's credentials
and change the data stored in the shared cache. When the first request
re-check credentials (for many reasons), they are changed. The change
can introduce a segfault.

This patch deactivate the cache upon success. When we need
authentication information from one request, they are re-parsed and
re-decoded. However, a failure to retrieve credentials is still
cached to avoid useless lookups.

This fix needs to be backported to 1.4 as well.
MINOR: doc: Bad cli function name.

The documentation describe a "disable" function,  but "enable" is
writed.
BUG/MEDIUM: http: fix regression caused by recent switch to keep-alive by default

Yesterday's commit 70dffda ("MAJOR: http: switch to keep-alive mode by default")
broke HTTP/1.0 handling without keep-alive when keep-alive is enabled both in
the frontend and in the backend.

Before this patch, it used to work because tunnel mode was the default one,
so if no mode was present in the frontend and a mode was set in the backend,
the backend was the first one to parse the header. This is what the original
patch tried to do with keep-alive by default, causing the version and the
connection header to be ignored if both the frontend and the backend were
running in keep-alive mode.

The fix consists in always parsing the header in non-tunnel mode, and
processing the rest of the logic in at least once, and again if the
backend works in a different mode than the frontend.

This is 1.5-specific, no backport is needed.
BUG/MEDIUM: listener: improve detection of non-working accept4()

On ARM, glibc does not implement accept4() and simply returns ENOSYS
which was not caught as a reason to fall back to accept(), resulting
in a spinning process since poll() would call again.

Let's change the error detection mechanism to save the broken status
of the syscall into a local variable that is used to fall back to the
legacy accept().

In addition to this, since the code was becoming a bit messy, the
accept4() was removed, so now the fallback code and the legacy code
are the same. This will also increase bug report accuracy if needed.

This is 1.5-specific, no backport is needed.
Merge branch 'master' of http://git.1wt.eu/git/haproxy