Add AuthBasicUseDigestAlgorithm directive to allow migration of
passwords from digest to basic authentication.

Proposed by: chrisd
Reviewed by: jim, rjung

Backport of r1514064 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1528957 13f79535-47bb-0310-9956-ffa450edef68