libflux/msg_handler: special case FLUX_ROLE_OWNER

Problem: it is confusing to have FLUX_ROLE_OWNER implicitly
set in the message handler rolemask when the user can't
manipulate that bit.

Take FLUX_ROLE_OWNER out of the rolemask and special case it
in call_handler() so it's more clear what is going on.

The net result is the same as before - FLUX_ROLE_OWNER is
always "allowed".

src/common/libflux/msg_handler.c

@@ -291,7 +291,7 @@ static void call_handler (flux_msg_handler_t *mh, const flux_msg_t *msg)
 
     if (flux_msg_get_rolemask (msg, &rolemask) < 0)
         return;
-    if (!(rolemask & mh->rolemask)) {
+    if (!(rolemask & FLUX_ROLE_OWNER) && !(rolemask & mh->rolemask)) {
         if (flux_msg_cmp (msg, FLUX_MATCH_REQUEST)
                         && flux_msg_get_matchtag (msg, &matchtag) == 0
                         && matchtag != FLUX_MATCHTAG_NONE) {
@@ -483,7 +483,6 @@ void flux_msg_handler_deny_rolemask (flux_msg_handler_t *mh, uint32_t rolemask)
 {
     if (mh) {
         mh->rolemask &= ~rolemask;
-        mh->rolemask |= FLUX_ROLE_OWNER;
     }
 }
 
@@ -532,7 +531,7 @@ flux_msg_handler_t *flux_msg_handler_create (flux_t *h,
     mh->magic = HANDLER_MAGIC;
     if (copy_match (&mh->match, match) < 0)
         goto nomem;
-    mh->rolemask = FLUX_ROLE_OWNER;
+    mh->rolemask = 0;
     mh->fn = cb;
     mh->arg = arg;
     mh->d = d;