MINOR: proxy: add a flag to memorize that the proxy's ID was forced

This will be used to know if proxy's ID should be considered when names
mismatch upon check status reload.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index cf3c093..1403bd1 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -2613,6 +2613,7 @@
 
 		curproxy->uuid = atol(args[1]);
 		curproxy->conf.id.key = curproxy->uuid;
+		curproxy->options |= PR_O_FORCED_ID;
 
 		if (curproxy->uuid <= 0) {
 			Alert("parsing [%s:%d]: custom id has to be > 0.\n",