[client] in '-M legacy' mode truncate the key to 16 bytes

This change helps to maintain backwards compatibility with older fwknopd daemons
that cannot handle Rijndael keys greater than 16 bytes.  Blair Zajac suggested
printing a warning in '-M legacy' mode when keys are attempted > 16 bytes long,
and this warning is included in this commit.
clarified NEWS file to state that fwknop is distributed under the GPL v2
[libfko] add ctx initialized check to fko_gpg_errstr()
[libfko] return proper GPG error code upon gpg_decrypt() failure
allow zero length to return FKO_SUCCESS from zero_buf() call
simplified zero_free() calls in support of #93