[libfko] Bug fix to not decrypt with GnuGP without FKO_ENC_MODE_ASYMMETRIC

[libfko] Bug fix to not attempt SPA packet decryption with GnuPG without
an fko object with encryption_mode set to FKO_ENC_MODE_ASYMMETRIC.  This
bug was caught with valgrind validation against the perl FKO extension
together with the set of SPA fuzzing packets in
test/fuzzing/fuzzing_spa_packets.  Note that this bug cannot be
triggered via fwknopd because additional checks are made within fwknopd
itself to force FKO_ENC_MODE_ASYMMETRIC whenever an access.conf stanza
contains GPG key information.  This fix strengthens libfko itself to
independently require that the usage of fko objects without GPG key
information does not result in attempted GPG decryption operations.  Hence
this fix applies mostly to third party usage of libfko - i.e. stock
installations of fwknopd are not affected.  As always, it is recommended to
use HMAC authenticated encryption whenever possible even for GPG modes since
this also provides a work around even for libfko prior to this fix.
minor ChangeLog rewording for GPG fix
move fuzzing_spa_packets file to perl/FKO/t/ for fuzzing tests

This change moves the fuzzing_spa_packets file from the test/fuzzing/
directory into the perl FKO extension t/ directory and is now referenced
directly by the t/04_fuzzing.t test file.  The test suite itself also uses
this file for fuzzing tests as well, but having the FKO built-in tests
enables Test::Valgrind memory checks so it is useful to have this included
in the FKO sources.  (When the FKO module is submitted to CPAN, it should
not depend on non-local files, but it's ok for the test suite to reference
the ../perl/FKO/t/ directory.)
[client] use libfko is_valid_ipv4_addr() for IP address validation
[test suite] added FreeBSD-9.2 and OpenBSD-5.4 compatibility tests