x509: rename a few variables and parameters

cn is not fully adequate as the name is not necessarily the Common Name.

Also, it's better have an explicit indication in the name of the variable if
it holds the expected name or some name from the certificate.
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index fe821d1..27662e1 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -274,7 +274,7 @@
  * \param crt      a certificate to be verified
  * \param trust_ca the trusted CA chain
  * \param ca_crl   the CRL chain for trusted CA's
- * \param cn       expected Common Name (can be set to
+ * \param exp_name expected Common Name (can be set to
  *                 NULL if the CN must not be verified)
  * \param flags    result of the verification
  * \param f_vrfy   verification function
@@ -290,7 +290,7 @@
 int mbedtls_x509_crt_verify( mbedtls_x509_crt *crt,
                      mbedtls_x509_crt *trust_ca,
                      mbedtls_x509_crl *ca_crl,
-                     const char *cn, uint32_t *flags,
+                     const char *exp_name, uint32_t *flags,
                      int (*f_vrfy)(void *, mbedtls_x509_crt *, int, uint32_t *),
                      void *p_vrfy );
 
@@ -308,7 +308,7 @@
  * \param trust_ca the trusted CA chain
  * \param ca_crl   the CRL chain for trusted CA's
  * \param profile  security profile for verification
- * \param cn       expected Common Name (can be set to
+ * \param exp_name expected Common Name (can be set to
  *                 NULL if the CN must not be verified)
  * \param flags    result of the verification
  * \param f_vrfy   verification function
@@ -325,7 +325,7 @@
                      mbedtls_x509_crt *trust_ca,
                      mbedtls_x509_crl *ca_crl,
                      const mbedtls_x509_crt_profile *profile,
-                     const char *cn, uint32_t *flags,
+                     const char *exp_name, uint32_t *flags,
                      int (*f_vrfy)(void *, mbedtls_x509_crt *, int, uint32_t *),
                      void *p_vrfy );