Use C functions for RC4 public API.

Use C functions for RC4 public API.

Rather than having public API switch between C and assembly, always
use C functions as entry points, which then call an assembly
implementation (if available). This makes it significantly easier
to deal with symbol aliasing/namespaces and it also means we
benefit from vulnerability prevention provided by the C compiler.

Rename the assembly generated functions from RC4() to rc4_internal()
and RC4_set_key() to rc4_set_key_internal(). Always include rc4.c
and change it to use defines that are similar to those used in BN.

ok beck@ joshua@ tb@