Nettle: Major cleanup and fixes of PCBC.
Nettle: Improved generic testing of ciphers.

Cipher modes may have different code paths for single blocks and
multiple blocks. They may also have problems with the empty string.
Make sure that the blocking factor doesn't affect the result.

FIXME: Ought to add similar code to the other generic tests.
Nettle: Added some tests of Crypto.AES.PCBC.

These tests use testvectors created by hand in Hilfe, as there doesn't
seem to be any public vectors anywhere.

Also adds note about alternate reading of PCBC in the
Kerberos v4 documentation.
sprintf: missing cyclic unlink in recursive _sprintf calls