sc: stream cipher abstraction applied to arcfour

A minor bug was discovered in padding of short message signatures. If is
silently fixed, causing some (very minor) incompatibility of signatures
with previous versions.
man: update FAQ
cosmetic fixes
chacha: new ChaCha20 stream cipher
chacha: fix discarding

although it's not needed here.
chacha: forgotten public
chacha: test vector compliance check
xsynd: new stream cipher

This one is especially cool. It is code based (yeyeye matches the
project name! :D ), it is provably secure (not like the others!) AAAND
is still quite fast (not fast as chacha though).