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/default_profile.h b/default_profile.h
index 58f619b..9c5bfb6 100644
--- a/default_profile.h
+++ b/default_profile.h
@@ -59,6 +59,7 @@
   static const char kStorageHostName[];
   static const char kStorageName[];
   static const char kStorageOfflineMode[];
+  static const char kStoragePortalCheckInterval[];
   static const char kStoragePortalURL[];
 
   const FilePath storage_path_;