Add an attach syslog

Adds an official attach syslog for better diagnostics in regular usage with
an attach approach.

Review-URL: https://codereview.appspot.com/319000044

core/dynamo.c

@@ -2748,6 +2748,13 @@ dynamorio_take_over_threads(dcontext_t *dcontext)
                "Failed to take over all threads after multiple attempts");
         ASSERT_NOT_REACHED();
     }
+    DO_ONCE({
+        char buf[16];
+        snprintf(buf, BUFFER_SIZE_ELEMENTS(buf), "%d", get_num_threads());
+        NULL_TERMINATE_BUFFER(buf);
+        SYSLOG(SYSLOG_INFORMATION, INFO_ATTACHED, 3, buf, get_application_name(),
+               get_application_pid());
+    });
 }
 
 /* Called by dynamorio_app_take_over in arch-specific assembly file */

core/win32/events.mc

@@ -125,6 +125,15 @@ Language=English
 Detaching from application %1!s! (%2!s!)
 .
 
+;// Info messages
+MessageId =
+Severity = Informational
+Facility = DRCore
+SymbolicName = MSG_INFO_ATTACHED
+Language=English
+Attached to %1!s! threads in application %2!s! (%3!s!)
+.
+
 ;//;;;;;;;;;;;;;;;;;;; Security
 
 MessageId =