Replace direct use of RC4 by stream cipher abstraction.

Replace direct use of RC4 by stream cipher abstraction.

This introduces the new struct stream_cipher in crypt.h as well as
two functions stream_cipher_new() and stream_cipher_free() which
initialize a new stream cipher structure and deallocate such a
structure, respectively.

The users of RC4 are changed to call the new abstract functions,
so they become independent from openssl. Consequently the affected
files need no include openssl/rc4.h any more.