Merge branch 'jn/5.1.1alpha+20120614'

Recent changes from upstream, including a documentation update and
small correctness fix for the "minimal version to decode" field.
Thanks to Lasse Collin for advice about which patches to apply.

After these changes, the xz and liblzma API and ABI match 5.1.2alpha,
except that no features that were not already present in Debian sid
have been added.