[MINOR] cfgparse: only keep one of dispatch, transparent, http_proxy

Since all of them are defined as proxy options, it's better to ensure
that at most one of them is enabled at once. The priority has been set
according to what is already performed in the backend :

   1) dispatch
   2) http_proxy
   3) transparent
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 8c23f94..18cfda3 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -5636,6 +5636,19 @@
 			}
 		}
 
+		if (curproxy->options2 & PR_O2_DISPATCH) {
+			curproxy->options  &= ~PR_O_TRANSP;
+			curproxy->options  &= ~PR_O_HTTP_PROXY;
+		}
+		else if (curproxy->options & PR_O_HTTP_PROXY) {
+			curproxy->options2 &= ~PR_O2_DISPATCH;
+			curproxy->options  &= ~PR_O_TRANSP;
+		}
+		else if (curproxy->options & PR_O_TRANSP) {
+			curproxy->options2 &= ~PR_O2_DISPATCH;
+			curproxy->options  &= ~PR_O_HTTP_PROXY;
+		}
+
 		if ((curproxy->options & PR_O_DISABLE404) && !(curproxy->options & PR_O_HTTP_CHK)) {
 			curproxy->options &= ~PR_O_DISABLE404;
 			Warning("config : '%s' will be ignored for %s '%s' (requires 'option httpchk').\n",