[server] minor error code text typo fixes
[libfko] implemented shared utility function for ipv4 address checking

This commit implements a single shared utility function for checking the
validaty of an IPv4 address, and both libfko and the fwknopd server use it
now.  The client will be updated as well.
[test suite] added short and long IP tests (1.1.1.1 and 123.123.123.123)
[test suite] better --lib-dir support for non-default LD_LIBRARY_PATH values

This commit allow the test suite to easily use a non-default LD_LIBRARY_PATH
in order to test mixed combinations of newly compiled fwknop client/server
binaries and previously installed versions of libfko.  This allows backwards
compatibility (and forwards compatibility) to be verified by the test suite.

For example, after compiling the fwknop client and server for the 2.5.2
release, one could use libfko from 2.5.1 to verify compatibility:

./test-fwknop.pl --enable-all --lib-path /home/mbr/install/fwknop-2.0.4/lib

See the --fwknop-path and --fwknopd-path args as well in order to support
arbitrary client/server/libfko combinations.