[server] minor replay code update to make functions static where possible
[server] fix crash if replay digest tracking init() fails

This commit fixes a crash if the replay digest init() routine fails - fwknopd
attempted to make use of replay tracking anyway.  The crash was discovered
during testing fwknopd with an AppArmor enforce policy deployed.  The
following stack trace shows the crash (taken before the previous static
function commit):

 Program received signal SIGSEGV, Segmentation fault.
 __strlen_sse2 () at ../sysdeps/x86_64/multiarch/../strlen.S:31
 31      ../sysdeps/x86_64/multiarch/../strlen.S: No such file or directory.
 (gdb) where
 #0  __strlen_sse2 () at ../sysdeps/x86_64/multiarch/../strlen.S:31
 #1  0x00007f59cabd8b26 in add_replay_file_cache (opts=opts@entry=0x7fff3eaa0bb0, digest=digest@entry=0x0) at replay_cache.c:516
 #2  0x00007f59cabd8cf5 in add_replay (opts=opts@entry=0x7fff3eaa0bb0, digest=digest@entry=0x0) at replay_cache.c:472
 #3  0x00007f59cabd62eb in incoming_spa (opts=0x7fff3eaa0bb0) at incoming_spa.c:536
 #4  0x00007f59ca56164e in ?? () from /usr/lib/x86_64-linux-gnu/libpcap.so.0.8
 #5  0x00007f59cabd7175 in pcap_capture (opts=opts@entry=0x7fff3eaa0bb0) at pcap_capture.c:269
 #6  0x00007f59cabd3d4d in main (argc=5, argv=0x7fff3eaa1458) at fwknopd.c:314
Added AppArmor policy

This commit adds an AppArmor policy that is known to work in Debian and Ubuntu
systems.  The original version of this policy was contributed by Radostan Riedel
to the fwknop mailing list.
added Radostan Riedel's AppArmor policy note