MINOR: net_helper: add sample converters to decode IP packet headers

MINOR: net_helper: add sample converters to decode IP packet headers

This adds a few converters that help decode parts of IP packets:
  - ip.data : returns the next header (typically TCP)
  - ip.df   : returns the dont-fragment flags
  - ip.dst  : returns the destination IPv4/v6 address
  - ip.hdr  : returns only the IP header
  - ip.proto: returns the upper level protocol (udp/tcp)
  - ip.src  : returns the source IPv4/v6 address
  - ip.tos  : returns the TOS / TC field
  - ip.ttl  : returns the TTL/HL value
  - ip.ver  : returns the IP version (4 or 6)

These can be used with the tcp-ss bind option. The doc was updated
accordingly.