Move EVP_Cipher() from evp_lib.c to evp_enc.c

Move EVP_Cipher() from evp_lib.c to evp_enc.c

EVP_Cipher() is a dangerous thin wrapper of the do_cipher() method set on
the EVP_CIPHER_CTX's cipher. It implements (part of) the update and final
step of the EVP_Cipher* API. Its behavior is nuts and will be documented
in a comment in a subsequent commit. schwarze has a manpage diff that will
fix the incorrect documentation.