Extend XWIKI-3013 (authenticate only once per session) to basic authentication

XWIKI-3013 introduce the proper usage of the session Principal in the form authentication mechanism with a configurable parameter. I feel that a lighter patch could permit to support both basic and form authentication to respond properly to this new feature with added benefit (especially in regards to LDAP authentication in basic mode).

Sorry to provide this in 3 patch, but this allow applying to either 1.7.2 and 1.8 as well.
For 1.8: apply partial-revert-3013.patch to revert partially XWIKI-3013
For 1.7: apply XWikiAuthServiceImpl.patch to apply partially XWIKI-3013
For both: apply MyBasicAuthenticator.patch to apply my new patch

Hope this helps.

Denis