libzmqutil: change auth log format used by broker

Problem: the broker logs a terse message displaying the certificate
name without adornment when a peer attempts to authenticate, but the
certificate name defaults to the name of the host that generated it,
which could easily be confused with the peer hostname.

Print cert-name=NAME instead of just the bare name to hopefully make
things a bit clearer.

Fixes #4341

Author: garlick

src/common/libzmqutil/zap.c

@@ -127,7 +127,11 @@ static void zap_cb (flux_reactor_t *r,
         }
         if (!name)
             name = "unknown";
-        logger (zap, log_level, "overlay auth %s %s", name, status_text);
+        logger (zap,
+                log_level,
+                "overlay auth cert-name=%s %s",
+                name,
+                status_text);
 
         if (!(rep = zmsg_new ()))
             goto done;