MEDIUM: proxy: replace proxy->state with proxy->disabled

The remaining proxy states were only used to distinguish an enabled
proxy from a disabled one. Due to the initialization order, both
PR_STNEW and PR_STREADY were equivalent after startup, and they
would only differ from PR_STSTOPPED when the proxy is disabled or
shutdown (which is effectively another way to disable it).

Now we just have a "disabled" field which allows to distinguish them.
It's becoming obvious that start_proxies() is only used to print a
greeting message now, that we'd rather get rid of. Probably that
zombify_proxy() and stop_proxy() should be merged once their
differences move to the right place.
diff --git a/contrib/prometheus-exporter/service-prometheus.c b/contrib/prometheus-exporter/service-prometheus.c
index ba1619f..b6c38eb 100644
--- a/contrib/prometheus-exporter/service-prometheus.c
+++ b/contrib/prometheus-exporter/service-prometheus.c
@@ -1546,12 +1546,12 @@
 			px = appctx->ctx.stats.obj1;
 
 			/* skip the disabled proxies, global frontend and non-networked ones */
-			if (px->state == PR_STSTOPPED || px->uuid <= 0 || !(px->cap & PR_CAP_FE))
+			if (px->disabled || px->uuid <= 0 || !(px->cap & PR_CAP_FE))
 				goto next_px;
 
 			switch (appctx->st2) {
 				case ST_F_STATUS:
-					metric = mkf_u32(FO_STATUS, px->state == PR_STREADY ? 1 : 0);
+					metric = mkf_u32(FO_STATUS, !px->disabled);
 					break;
 				case ST_F_SCUR:
 					metric = mkf_u32(0, px->feconn);
@@ -1731,7 +1731,7 @@
 			px = appctx->ctx.stats.obj1;
 
 			/* skip the disabled proxies, global frontend and non-networked ones */
-			if (px->state == PR_STSTOPPED || px->uuid <= 0 || !(px->cap & PR_CAP_BE))
+			if (px->disabled || px->uuid <= 0 || !(px->cap & PR_CAP_BE))
 				goto next_px;
 
 			switch (appctx->st2) {
@@ -1976,7 +1976,7 @@
 			px = appctx->ctx.stats.obj1;
 
 			/* skip the disabled proxies, global frontend and non-networked ones */
-			if (px->state == PR_STSTOPPED || px->uuid <= 0 || !(px->cap & PR_CAP_BE))
+			if (px->disabled || px->uuid <= 0 || !(px->cap & PR_CAP_BE))
 				goto next_px;
 
 			while (appctx->ctx.stats.obj2) {