Constify d2i, s2i, c2i and r2i functions and other associated
functions and macros.

This change has associated tags: LEVITTE_before_const and
LEVITTE_after_const.  Those will be removed when this change has been
properly reviewed.
diff --git a/ssl/ssl_locl.h b/ssl/ssl_locl.h
index 4adf5f5..c2e6ff8 100644
--- a/ssl/ssl_locl.h
+++ b/ssl/ssl_locl.h
@@ -544,7 +544,7 @@
 int ssl2_put_cipher_by_char(const SSL_CIPHER *c,unsigned char *p);
 int ssl2_part_read(SSL *s, unsigned long f, int i);
 int ssl2_do_write(SSL *s);
-int ssl2_set_certificate(SSL *s, int type, int len, unsigned char *data);
+int ssl2_set_certificate(SSL *s, int type, int len, const unsigned char *data);
 void ssl2_return_error(SSL *s,int reason);
 void ssl2_write_error(SSL *s);
 int ssl2_num_ciphers(void);