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/dbus_bindings/org.chromium.flimflam.Manager.xml b/dbus_bindings/org.chromium.flimflam.Manager.xml
index 0047c5c..ff4c041 100644
--- a/dbus_bindings/org.chromium.flimflam.Manager.xml
+++ b/dbus_bindings/org.chromium.flimflam.Manager.xml
@@ -34,6 +34,7 @@
 			<arg type="s" direction="in"/>
 		</method>
 		<method name="PopAnyProfile"/>
+		<method name="RecheckPortal"/>
 		<method name="RequestScan">
 			<arg type="s" direction="in"/>
 		</method>