MINOR: checks: Add a way to send custom headers and payload during http chekcs

MINOR: checks: Add a way to send custom headers and payload during http chekcs

The 'http-check send' directive have been added to add headers and optionnaly a
payload to the request sent during HTTP healthchecks. The request line may be
customized by the "option httpchk" directive but there was not official way to
add extra headers. An old trick consisted to hide these headers at the end of
the version string, on the "option httpchk" line. And it was impossible to add
an extra payload with an "http-check expect" directive because of the
"Connection: close" header appended to the request (See issue #16 for details).

So to make things official and fully support payload additions, the "http-check
send" directive have been added :

    option httpchk POST /status HTTP/1.1

    http-check send hdr Content-Type "application/json;charset=UTF-8" \
        hdr X-test-1 value1 hdr X-test-2 value2 \
        body "{id: 1, field: \"value\"}"

When a payload is defined, the Content-Length header is automatically added. So
chunk-encoded requests are not supported yet. For now, there is no special
validity checks on the extra headers.

This patch is inspired by Kiran Gavali's work. It should fix the issue #16 and
as far as possible, it may be backported, at least as far as 1.8.

(cherry picked from commit 8acb1284bcd47301fce0aa45a7083dbcef94cb5f)
Signed-off-by: Willy Tarreau <w@1wt.eu>