Add and use a constant-time memcmp.

This change adds CRYPTO_memcmp, which compares two vectors of bytes in
an amount of time that's independent of their contents. It also changes
several MAC compares in the code to use this over the standard memcmp,
which may leak information about the size of a matching prefix.
(cherry picked from commit 2ee798880a246d648ecddadc5b91367bee4a5d98)

crypto/cryptlib.c

diff --git a/crypto/cryptlib.c b/crypto/cryptlib.c
index 07b0a66..56d82ad 100644
--- a/crypto/cryptlib.c
+++ b/crypto/cryptlib.c
@@ -397,3 +397,16 @@
 #ifndef OPENSSL_FIPSCANISTER
 void *OPENSSL_stderr(void)	{ return stderr; }
 #endif
+
+int CRYPTO_memcmp(const void *in_a, const void *in_b, size_t len)
+	{
+	size_t i;
+	const unsigned char *a = in_a;
+	const unsigned char *b = in_b;
+	unsigned char x = 0;
+
+	for (i = 0; i < len; i++)
+		x |= a[i] ^ b[i];
+
+	return x;
+	}