Add a way to get all values of an attribute of DN.

This is needed to switch Apache HTTP hostname verification from its
own Distinguished Name (DN) parsing code to this library's
DistinguishedNameParser.

Bug: 16510257

(cherry picked from commit 7bbe7cb0a77527a8d4043315f70c156b2447b462)

Change-Id: I4a20729dad557aebe0bd055736777e34b51167e8