netlink: add netlink user documentation.

Add netlink(4) as a "frontend" manpage describing netlink in general.
Add rtnelink(4) describing supported commands and attributes in
NETLINK_ROUTE family.
Add genetlink(4) describing generic netlink API.

Reviewed by: pauamma
Differential Revision: https://reviews.freebsd.org/D37011

(cherry picked from commit 7366c0a49c9a60d3eea7520d7ae4bc2b3ab172f3)