shill: wimax: Don't ask users to re-enter credentials if available.

shill now sets the PassphraseRequired to true only if credentials are
not available, or if a connect attempt to a live network failed. Also,
refuse connecting to a service that's marked as non-connectable, and
fix UpdateConnectable in WiMaxService::Start.

BUG=chrome-os-partner:10115
TEST=unit tests

Change-Id: I53989736a82e4062422c648c58ccc83abc045a86
Reviewed-on: https://gerrit.chromium.org/gerrit/24187
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
diff --git a/wimax_service.h b/wimax_service.h
index 44c5ef8..896bb97 100644
--- a/wimax_service.h
+++ b/wimax_service.h
@@ -63,6 +63,8 @@
   static std::string CreateStorageIdentifier(const WiMaxNetworkId &id,
                                              const std::string &name);
 
+  virtual void ClearPassphrase();
+
   // Inherited from Service.
   virtual bool TechnologyIs(const Technology::Identifier type) const;
   virtual void Connect(Error *error);