BUG/MINOR: backend: make be_usable_srv() consistent when stopping

When a proxy enters the STOPPED state, it will no longer accept new
connections.

However, it doesn't mean that it's completely inactive yet: it will
still be able to handle already pending / keep-alive connections,
thus finishing ongoing work before effectively stopping.

be_usable_srv(), which is used by nbsrv converter and sample fetch,
will return 0 if the proxy is either stopped or disabled.

nbsrv behaves this way since it was originally implemented in b7e7c4720
("MINOR: Add nbsrv sample converter").

(Since then, multiple refactors were performed around this area, but
the current implementation still follows the same logic)

It was found that if nbsrv is used in a proxy section to perform routing
logic, unexpected decisions are being made when nbsrv is used on a proxy
with STOPPED state, since in-flight requests will suffer from nbsrv
returning 0 instead of the current number of usable servers which may
still process existing connections.
For instance, this can happen during process soft-stop, or even when
stopping the proxy from the cli / lua.

To fix this: we now make sure be_usable_srv() always returns the
current number of usable servers, unless the proxy is explicitly
disabled (from the config, not at runtime)

This could be backported up to 2.6.
For older versions, the need for a backport should be evaluated first.

--
Note for 2.4: proxy flags did not exist, it was implemented with fd10ab5e
("MINOR: proxy: Introduce proxy flags to replace disabled bitfield")

For 2.2: STOPPED and DISABLED states were not separated, so we have no
easy way to apply the fix anyway.

include/haproxy/backend.h

@@ -51,7 +51,7 @@ int be_lastsession(const struct proxy *be);
 /* Returns number of usable servers in backend */
 static inline int be_usable_srv(struct proxy *be)
 {
-        if (be->flags & (PR_FL_DISABLED|PR_FL_STOPPED))
+        if (be->flags & PR_FL_DISABLED)
                 return 0;
         else if (be->srv_act)
                 return be->srv_act;