route/xfrmi: merge branch 'ebirger/add-xfrmi-support'

Rebased original pull request #214 on master. One commit ("xfrmi: add
IFLA_XFRM_* definitions to if_link.h") was thereby dropped as it
is no longer necessary. Otherwise, the two remaining patches applied
cleanly. Then, add two more patches to the branch before merging.

https://github.com/thom311/libnl/pull/214