shill: portal: Recheck portal state

Introduce a retry interval for automatically retrying portal
checks.  Also provide a Manager API method for immediately
re-checking portal status.

BUG=chromium-os:27335
TEST=New unit tests, tested on real machine, including setting
PortaCheckInterval over DBus, and using Jason's addition to
test-flimflam for 'recheck-portal'.
Change-Id: Idc7def18c6f863859e94f4d4e9f266ab2670679c
Reviewed-on: https://gerrit.chromium.org/gerrit/17367
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/manager_dbus_adaptor.cc b/manager_dbus_adaptor.cc
index b7c283b..a86367c 100644
--- a/manager_dbus_adaptor.cc
+++ b/manager_dbus_adaptor.cc
@@ -135,6 +135,12 @@
   e.ToDBusError(&error);
 }
 
+void ManagerDBusAdaptor::RecheckPortal(::DBus::Error &error) {
+  Error e;
+  manager_->RecheckPortal(&e);
+  e.ToDBusError(&error);
+}
+
 void ManagerDBusAdaptor::RequestScan(const string &technology,
                                      ::DBus::Error &error) {
   Error e;