[test suite] use digest tracking override for ALF fwknopd fuzzing
[server] Enforce proper bounds checking on digest cache file import

Bug fix to ensure that proper bounds are enforced when importing digest
cache files from previous fwknopd executions. This bug
was discovered through fuzzing with American Fuzzy Lop (AFL) as driven
by the test/afl/fuzzing-wrappers/server-digest-cache.sh wrapper.
Previous to this fix, fwknopd could be made to crash through a malicious
digest cache file (normally in /var/run/fwknop/digest.cache) upon
initial import.
[server] add --run-dir command line arg
[test suite] add AFL wrapper for fuzzing digest cache file format