arcfour: greatly increase discard parameters

Okay, this thing got public so it's time to make the RC4 rugged. Not
that I'd know about something that would break current implementation,
but it's nice to at least do the recommended discard correctly.

We'll probably be adding better symmetric ciphers anyway.

Note that this is an incompatible change (again). FMTSeq private keys
will need to be replaced. Existing signature validity doesn't change.
Encrypted messages will not be possible to decrypt.
manual page
sha2: fix the ugly warning

memcpy() is better than dereferencing a type-punned-pointer for
simulating memcpy()
algos_sig: added H=20 fmtseq variant

...that provide around 1 million signatures per key. That seems more
than sufficient for human usage.

Note that generating the key takes 16 times longer than for already
present algorithms (that have H=16). On my computer, it is around 4
minutes for fmtseq128N20 and 16 minutes for fmtseq256N20.
upgrade docs after algo names changed
forgotten styleup
fmtseq: forgotten include