samples: pktgen: add some helper functions for port parsing

This commit adds port parsing and port validate helper function to parse
single or range of port(s) from a given string. (e.g. 1234, 443-444)

Helpers will be used in prior to set target port(s) in samples/pktgen.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>