Initial structure for supervisor-mode

The full process hierarchy isn't set up yet, but these changes
help prepare by doing two things:

- Add a -j option to enable supervisor-mode.  Currently, just a single
  "stem" process gets forked early on to be used as the basis for
  further forking into real cluster nodes.

- Separates the parsing of command-line options from their consumption.
  i.e. need to parse whether we're in -j supervisor-mode before
  modifying any global state since that would taint the "stem" process.
  The new intermediate structure containing the parsed options may
  also serve as a way to pass configuration info from "stem" to its
  descendent cluster node processes.

src/util.cc

@@ -1003,7 +1003,7 @@ string bro_prefixes()
 	{
 	string rval;
 
-	for ( const auto& prefix : prefixes )
+	for ( const auto& prefix : zeek_script_prefixes )
 		{
 		if ( ! rval.empty() )
 			rval.append(":");
@@ -1936,3 +1936,18 @@ string json_escape_utf8(const string& val)
 
 	return result;
 	}
+
+void zeek::set_thread_name(const char* name, pthread_t tid)
+	{
+#ifdef HAVE_LINUX
+	prctl(PR_SET_NAME, name, 0, 0, 0);
+#endif
+
+#ifdef __APPLE__
+	pthread_setname_np(name);
+#endif
+
+#ifdef __FreeBSD__
+	pthread_set_name_np(tid, name);
+#endif
+	}