ofp-parse: Fix parsing, formatting of multiple fields in NTR extension.

ofp-parse: Fix parsing, formatting of multiple fields in NTR extension.

Until now, the only way to specify multiple fields in the "fields"
parameter for the Netronome groups extension, was to specify "fields"
more than once, e.g. fields=eth_dst,fields=ip_dst

However, this wasn't documented and the code in ofp-print didn't use it,
generating output that couldn't be parsed.

This commit fixes the situation by introducing a more straightforward
syntax, e.g. fields(eth_dst,ip_dst), documents it, and adjusts ofp-print
code to use it when there is more than one field (it retains the previous
format for backward compatibility when there is exactly one field)

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Simon Horman <simon.horman@netronome.com>