broker: register content attributes early

Occasionally this sharness test fails:

not ok 5 - Attempt to start instance with invalid hash fails hard
FAIL: t2008-althash.t 5 - Attempt to start instance with invalid hash fails hard
 #
 #  test_must_fail flux start -o,-Scontent.hash=wronghash /bin/true
 #
The broker asserts in zsys teardown:
 src/zsock_option.c:1347: zsock_set_sndtimeo:
   Assertion `rc == 0 || zmq_errno () == (156384712 + 53)' failed.

Since we are deliberately triggering this early failure in sharness,
move the content-cache attribute initialization to before
zsys is initialized.

Fixes flux-framework#1063

src/broker/broker.c

@@ -365,6 +365,9 @@ int main (int argc, char *argv[])
             log_err_exit ("flux_open enclosing instance");
     }
 
+    if (content_cache_register_attrs (ctx.cache, ctx.attrs) < 0)
+        log_err_exit ("content cache attributes");
+
     /* Block all signals, saving old mask and actions for SIGINT, SIGTERM.
      */
     sigset_t sigmask;
@@ -499,8 +502,7 @@ int main (int argc, char *argv[])
             || attr_add_active_int (ctx.attrs, "tbon.descendants",
                                 &ctx.tbon.descendants,
                                 FLUX_ATTRFLAG_IMMUTABLE) < 0
-            || hello_register_attrs (ctx.hello, ctx.attrs) < 0
-            || content_cache_register_attrs (ctx.cache, ctx.attrs) < 0) {
+            || hello_register_attrs (ctx.hello, ctx.attrs) < 0) {
         log_err_exit ("configuring attributes");
     }