[server] zero out access stanza key information before exit (in support of #93)
[server] minor header formating update
[server] update fw_config_init() to allow access stanza key information to be zeroed out upon error (#93)
continued changes to zero out sensitive information before exit (#93)
continued zeroing out of sensitive data buffers in support of issue #93
[client] make legacy encryption mode and HMAC usage mutually exclusive
[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
[client] minor man page update to state that -a is more secure than -R
[libfko] bug fix to set digest length upon SPA packet decode

This bug was caught with the fko_wrapper.c multi-call tester running under
valgrind.
[libfko] always call free() from zero_free() on all non-NULL buf pointers
[libfko] use zero_free_rv - dead code bug fix found by CLANG static analyzer
[server] compile bug fix for pf/ipfw firewall systems
minor README typo fixes
[libfko] apply zero_buf() to stack allocated Rijndael context for encrypt/decrypt
[server] Account for older versions of iptables that don't have -C

This commit updates fwknopd to test for the existance of the iptables '-C'
rule checking functionality since older versions of iptables don't have this.
If it isn't offered by the installed version of iptables, then revert to parsing
fwknop chains to see if iptables rules already exist before adding new rules (to
avoid duplicates).