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/Baseline/scripts.base.utils.active-http/bro..stdout

@@ -0,0 +1,5 @@
+[code=200, msg=OK^M, body=It works!, headers={
+[Server] =  1.0,
+[Content-type] =  text/plain,
+[Date] =  July 22, 2013
+}]