src: add support for display maps content

src: add support for display maps content

This commit adds a new command that displays the definition of a single
map:

  # nft list map [family] <table> <map>

If no family is specified, ip is assumed.

Example:

  # nft list map ip6 filter test

  table ip6 filter {
          map test {
                  type ipv6_addr : inet_service
                  elements = { 2001:db8::ff00:42:8329 : http}
          }
  }

Signed-off-by: Pablo M. Bermudo Garay <pablombg@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>