[test suite] call FKO functions via function pointers (interim commit)
[test suite] added ctx_update() function to fko-wrapper test
[test suite] update fko-wrapper to use constants from fko.h
[test suite] added 'getset' versions of fko_ int/short wrapper functions
[test suite] use ctx_update() where possible for fko-wrapper
[test suite] run fko-wrapper without valgrind, closes #113
added fko-wrapper no valgrind script
[libfko] reject negative length values

Integer lengths that are negative are never valid.  This commit also
extends the fuzzing capabilities of the test/fko-wrapper code to
validate libfko calls with negative length arguments, and one crash
scenario with a negative length for the encryption key was found (and
fixed) this way.