netlink: allow creating sockets with SOCK_DGRAM.

[melifaro]

Summary

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.

[melifaro]

Add tests.

[pauamma_gundo.com]

Manual page change LGTM. Can't attest to consistency with source code.