examples/ipsec-secgw: use POSIX network address conversion

examples/ipsec-secgw: use POSIX network address conversion

inet_pton4 and inet_pton6 was reimplemented. Replace implementation of
inet_pton4 and inet_pton6 with libc inet_pton function

Bugzilla ID: 365
Fixes: 0d547ed03717 ("examples/ipsec-secgw: support configuration file")
Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Ibtisam Tariq <ibtisam.tariq@emumba.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: David Marchand <david.marchand@redhat.com>