cmdline: standardize conversion of IP address strings

cmdline: standardize conversion of IP address strings

The code to convert IPv4 and IPv6 address strings into a binary format
(inet_ntop) was included in the cmdline library. This was needed when
the DPDK was compiled in environments where the standard inet_ntop()
function is not available. Today, this is not the case and the standard
inet_ntop() is always available.

This patch removes the internal inet_ntop() functions and the specific
license.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>