Merge branch 'master' into 'master'

choose better MAC for squeeze and wheezy

both squeeze (1:5.5p1-6+squeeze6) and wheezy (1:6.0p1-4+deb7u2) have MACs better than hmac-sha1 available in the default search, they both have hmac-sha2-512, hmac-sha2-256, and hmac-ripemd160. So switch to using hmac-sha2-512, which lets us lock down the client MACs more.

See merge request !19