[BUG] reserve some pipes for backends with splice enabled

If splicing is enabled in a backend, we need to guess how many
pipes will be needed. We used to rely on fullconn, but this leads
to non-working splicing when fullconn is not specified. So we now
fallback to global.maxconn.
diff --git a/src/haproxy.c b/src/haproxy.c
index 5911b0c..9b83146 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -560,7 +560,7 @@
 				if (cur->cap & PR_CAP_FE)
 					nbfe += cur->maxconn;
 				if (cur->cap & PR_CAP_BE)
-					nbbe += cur->fullconn;
+					nbbe += cur->fullconn ? cur->fullconn : global.maxconn;
 			}
 		}
 		global.maxpipes = MAX(nbfe, nbbe);