app/testpmd: add ability to set Tx IP and UDP parameters

app/testpmd: add ability to set Tx IP and UDP parameters

This patch changes what testpmd uses as IP addresses when
run in transmit only mode. The old code was using
192.168.0.1 -> 192.168.0.2
but these addresses are reserved for private Internet by RFC 1918.

The new code uses 192.18.0.1 and 192.18.0.2 which are on the
subnet reserved for performance testing by RFC 2544.

New command line option allows the user to pick any other src/dst
address desired.

Notice: this changes the default IP address for transmit only.
It may cause some user who has hardcoded network addresses to report
a regression.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>