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/mock_manager.h b/mock_manager.h
index 5b97d96..32f0e45 100644
--- a/mock_manager.h
+++ b/mock_manager.h
@@ -32,6 +32,7 @@
                      const std::string &entry_name));
   MOCK_METHOD1(IsPortalDetectionEnabled, bool(Technology::Identifier tech));
   MOCK_CONST_METHOD0(GetPortalCheckURL, const std::string &());
+  MOCK_CONST_METHOD0(GetPortalCheckInterval, int());
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MockManager);