linuxolator: add netlink support

Add the glue code to support netlink in Linuxolator.
linux_common(4) now depends on netlink(4).

All netlink protocol constants are consistent with the Linux version.
However, certain OS-specific constants such as AF_INET6, interface
flags or default routing table id, are different between FreeBSD and
Linux. Thus, it may be needed to rewrite some message parts or even
rewrite the whole message, adding or removing some TLVs. The core
netlink implementation code provides efficient rewriting callbacks
which Linuxolator now uses.

Reviewed by: dchagin
Differential Revision: https://reviews.freebsd.org/D36361
MFC after: 2 months