Exec module changes/fixes.

- Give Dir::monitor() a param for the polling interval, so different
  dirs can be monitored at different frequencies.

- Fix race in Exec::run() when reading extra output files produced by
  a process -- it was possible for Exec::run() to return before all
  extra output files had been fully read.

- Add test cases.

testing/btest/scripts/base/utils/active-http.test

@@ -0,0 +1,25 @@
+# @TEST-EXEC: btest-bg-run httpd python $SCRIPTS/httpd.py --max 1
+# @TEST-EXEC: sleep 3
+# @TEST-EXEC: btest-bg-run bro bro -b %INPUT
+# @TEST-EXEC: btest-bg-wait 15
+# @TEST-EXEC: btest-diff bro/.stdout
+
+@load base/utils/active-http
+
+redef exit_only_after_terminate = T;
+
+event bro_init()
+	{
+	local req = ActiveHTTP::Request($url="localhost:32123");
+
+	when ( local resp = ActiveHTTP::request(req) )
+		{
+		print resp;
+		terminate();
+		}
+	timeout 1min
+		{
+		print "HTTP request timeout";
+		terminate();
+		}
+	}