Incorporate the LGPL v2.1 licensed implementation of ARC4, taken from
the Nettle project (https://git.lysator.liu.se/nettle/nettle.git,
commit 3e7a480a1e351884), and tweak it a bit so we don't have to
operate on a skip buffer to fast forward the stream cipher, but can
simply invoke it with NULL dst or src arguments to achieve the same.
This removes the dependency [via libell] on the OS's implementation of
ecb(arc4), which may be going away, and which is not usually accelerated
in the first place.
1db8a85a60