Http auth select

I have added the ability for the user to choose between DIGEST and BASIC authentication for WebDAV. My institution would not permit use of storage that has user choose a separate password for personal file access.

I'm not thrilled with the password caching mechanism I'm using, but every file action in WebDAV was triggering full ldap_bind authentication (session credentials are not stored for the default WebDAV capabilities of Windows).

I'm still getting used to using github, so let me know if I messed something up.