[libfko] move is_base64 check to libfko

This commit replaces the separately implemented client/server
is_base64() check with a single libfko function, and libfko itself now
uses it as well before prepending Rijndael or GnuPG base64 encoded
prefixes.
[test suite] minor addition to fko-wrapper to call fko_errstr() across valid and invalid values
[test suite] added tests/code_structure.pl with a test for expected lib/fko.h error code fko_errstr() handling
[libfko] bug fix caught by new code structure error str test to add string for FKO_ERROR_INVALID_DATA_DECODE_EXTRA_TOOBIG