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/src/extcheck.c b/src/extcheck.c
index dd7c942..96d61c9 100644
--- a/src/extcheck.c
+++ b/src/extcheck.c
@@ -495,7 +495,7 @@
 		 * is disabled.
 		 */
 		if (((check->state & (CHK_ST_ENABLED | CHK_ST_PAUSED)) != CHK_ST_ENABLED) ||
-		    s->proxy->state == PR_STSTOPPED)
+		    s->proxy->disabled)
 			goto reschedule;
 
 		/* we'll initiate a new check */