Major restructuring of the internals of SILC Cipher API

The API now has two new operation functions init and uninit, and the
old context_len operation is removed.  The init is called in
silc_cipher_alloc and uninit is called in silc_cipher_free.  The init()
function must allocate and return the internal algorithm context.
Changed all supported ciphers to use the new API.

Fixed also CTR mode encryption of partial blocks.  Optimized also the CTR
mode especially on little endian systems.

Imported entirely new Twofish implementation from the public domain
library libtomcrypt.

Renamed rjindael_internal.h to aes_internal.h.