[libfko] HMAC comparison timing bug fix

Ryman reported a timing attack bug in the HMAC comparison operation (#85) and
suggested a fix derived from YaSSL:
http://www.mail-archive.com/debian-bugs-rc@lists.debian.org/msg320402.html
Convert strncmp() calls to constant_runtime_cmp() at various places

This commit is a follow up to Ryman's report (#85) of a potential timing attack
that could be leveraged against fwknop when strncmp() is used to compare HMAC
digests.  All strncmp() calls that do similar things have been replaced with a
new constant_runtime_cmp() function that mitigates this problem.
added fwknopd man page blurb for the ENABLE_PCAP_ANY_DIRECTION variable
Merge branch 'hmac_timing_bug_fix'

Fixes #85
ChangeLog update to mention the constant_runtime_cmp() change