RT1815: More const'ness improvements

Add a dozen more const declarations where appropriate.
These are from Justin; while adding his patch, I noticed
ASN1_BIT_STRING_check could be fixed, too.

Reviewed-by: Dr. Stephen Henson <steve@openssl.org>

crypto/asn1/asn1.h

diff --git a/crypto/asn1/asn1.h b/crypto/asn1/asn1.h
index d6342d0..925fdba 100644
--- a/crypto/asn1/asn1.h
+++ b/crypto/asn1/asn1.h
@@ -819,9 +819,9 @@
 int		ASN1_BIT_STRING_set(ASN1_BIT_STRING *a, unsigned char *d,
 			int length );
 int		ASN1_BIT_STRING_set_bit(ASN1_BIT_STRING *a, int n, int value);
-int		ASN1_BIT_STRING_get_bit(ASN1_BIT_STRING *a, int n);
-int            ASN1_BIT_STRING_check(ASN1_BIT_STRING *a,
-                                     unsigned char *flags, int flags_len);
+int		ASN1_BIT_STRING_get_bit(const ASN1_BIT_STRING *a, int n);
+int            ASN1_BIT_STRING_check(const ASN1_BIT_STRING *a,
+                                     const unsigned char *flags, int flags_len);
 
 #ifndef OPENSSL_NO_BIO
 int ASN1_BIT_STRING_name_print(BIO *out, ASN1_BIT_STRING *bs,