shill: Clear PMKSA cached credentials on unload

When an 802.1x WiFi service Unload()s, ask wpa_supplicant to clear
all cached credentials.  This is a workaround for dropping PMKSA
cache entries in wpa_supplicant when the user logs ouut.  Newer
versions of wpa_supplicant will support dropping cache entries
automatically when netblocks are changed.  This will allow more
granular key clearing.

BUG=chromium-os:23367
TEST=New unit test

Change-Id: I94d69b6943d69c7f5f31ed7846d908b38ce8e6c8
Reviewed-on: https://gerrit.chromium.org/gerrit/15163
Commit-Ready: Paul Stewart <pstew@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/supplicant_interface_proxy.h b/supplicant_interface_proxy.h
index a021175..5d4d32f 100644
--- a/supplicant_interface_proxy.h
+++ b/supplicant_interface_proxy.h
@@ -29,6 +29,7 @@
 
   virtual ::DBus::Path AddNetwork(
       const std::map<std::string, ::DBus::Variant> &args);
+  virtual void ClearCachedCredentials();
   virtual void Disconnect();
   virtual void FlushBSS(const uint32_t &age);
   virtual void RemoveAllNetworks();