erspan: add erspan version II support

erspan: add erspan version II support

The patch adds support for configuring the erspan v2, for both
ipv4 and ipv6 erspan implementation.  Three additional fields
are added: 'erspan_ver' for distinguishing v1 or v2, 'erspan_dir'
for specifying direction of the mirrored traffic, and 'erspan_hwid'
for users to set ERSPAN engine ID within a system.

As for manpage, the ERSPAN descriptions used to be under GRE, IPIP,
SIT Type paragraph.  Since IP6GRE/IP6GRETAP also supports ERSPAN,
the patch removes the old one, creates a separate ERSPAN paragrah,
and adds an example.

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>