netlink: allow creating sockets with SOCK_DGRAM.

Some existing applications setup Netlink socket with
SOCK_DGRAM instead of SOCK_RAW. Update the manpage to clarify
that the default way of creating the socket should be with
SOCK_RAW. Update the code to support both SOCK_RAW and SOCK_DGRAM.

Reviewed By: pauamma
Differential Revision: https://reviews.freebsd.org/D38075

(cherry picked from commit 0079d177ab69168df5e103128a2b15bd8e74d4eb)