netlink: indentation & whitespace/tab consistency
netlink: comments & cleanup
netlink: __attribute__ usage clarification

As requested by @octo in GitHub#376.
update README to mention libmnl dependency
update build system to reflect switch to libmnl

"./configure" was previously checking for libnetlink and setting
"-lmnl" if found, which was obviously calling for trouble. With
this patch, the last references to libnetlink are dead and buried.

NB: linux/netlink.h and friends are still included here and there
though, but these are not related to libnetlink.