Fully support X509 certificates for ActiveSync.

Allow separate configuration for ActiveSync Authentication methods.
Emulates Exchange server's ability to accept either: HTTP Basic only,
client certificate only, or to require both HTTP Basic AND client
certificates. If configured to require both, horde-wide Auth driver
is used to authenticate using the HTTP Basic credentials, and the
X509 driver is used to to authenticate with the client certificate.
Obviously requires webserver config/support for the certificates.
This belongs in Horde_5_2
This config isn't available until Horde_5_2, existence check it.
CHANGES