Make rc4 encryption/decryption more explicit.

Make rc4 encryption/decryption more explicit.

The old code used a rather fancy way to (de)activate encryption on
a per fd basis: In net.c there was an array of rc4 keys, indexed by fd.
This is rather wastful because the array size is determined by the
highest fd for which encryption is activated.

It's also hard to comprehend and it's easy to get strange results if
one forgets to deactivate the encryption after the fd is closed.

This patch introduces a new structure, rc4_context, which consists of
an fd and the needed rc4 keys. Users explicitly call the new rc4 variants
of the receving/sending functions which take a pointer to a struct
rc4_context instead of a file descriptor but are otherwise identical.

Keep crypt simple!