MAJOR: check: find out which port to use for health check at run time

HAProxy used to deduce port used for health checks when parsing configuration
at startup time.
Because of this way of working, it makes it complicated to change the port at
run time.

The current patch changes this behavior and makes HAProxy to choose the
port used for health checking when preparing the check task itself.

A new type of error is introduced and reported when no port can be found.

There won't be any impact on performance, since the process to find out the
port value is made of a few 'if' statements.

This patch also introduces a new check state CHK_ST_PORT_MISS: this flag is
used to report an error in the case when HAProxy needs to establish a TCP
connection to a server, to perform a health check but no TCP ports can be
found for it.

And last, it also introduces a new stream termination condition:
SF_ERR_CHK_PORT. Purpose of this flag is to report an error in the event when
HAProxy has to run a health check but no port can be found to perform it.
diff --git a/include/types/stream.h b/include/types/stream.h
index 17e74b8..0d8c500 100644
--- a/include/types/stream.h
+++ b/include/types/stream.h
@@ -73,6 +73,7 @@
 #define SF_ERR_DOWN     0x00009000	/* the proxy killed a stream because the backend became unavailable */
 #define SF_ERR_KILLED   0x0000a000	/* the proxy killed a stream because it was asked to do so */
 #define SF_ERR_UP       0x0000b000	/* the proxy killed a stream because a preferred backend became available */
+#define SF_ERR_CHK_PORT 0x0000c000	/* no port could be found for a health check. TODO: check SF_ERR_SHIFT */
 #define SF_ERR_MASK     0x0000f000	/* mask to get only stream error flags */
 #define SF_ERR_SHIFT    12		/* bit shift */