Update the PLAIN, CRAM-MD5, and DIGEST-MD5 SASL mechanism handlers to use the
password policy during the process of verifying the credentials rather than
just assuming that the password is held in the userPassword attribute.

Also, fix a bug in the DIGEST-MD5 implementation that prevented authentication
from completing successfully when the "u:" form of the authentication ID was
provided.

OpenDS Issue Numbers:  698, 699