[MEDIUM] checks: support multi-packet health check responses

[MEDIUM] checks: support multi-packet health check responses

We are seeing both real servers repeatedly going on- and off-line with
a period of tens of seconds. Packet tracing, stracing, and adding
debug code to HAProxy itself has revealed that the real servers are
always responding correctly, but HAProxy is sometimes receiving only
part of the response.

It appears that the real servers are sending the test page as three
separate packets. HAProxy receives the contents of one, two, or three
packets, apparently randomly. Naturally, the health check only
succeeds when all three packets' data are seen by HAProxy. If HAProxy
and the real servers are modified to use a plain HTML page for the
health check, the response is in the form of a single packet and the
checks do not fail.

(...)
I've added buffer and length variables to struct server, and allocated
space with the rest of the server initialisation.

(...)
It seems to be working fine in my tests, and handles check responses
that are bigger than the buffer.