Give x509_{sequence|name}_free() external linkage

With the introduction of `mbedtls_x509_crt_get_{issuer|name}()`,
users need an easy way of freeing the dynamic name structures these
functions return.

To that end, this commit renames `x509_{sequence|name}_free()`
to `mbedtls_x509_{sequence|name}_free()` and gives them external linkage.
diff --git a/include/mbedtls/x509.h b/include/mbedtls/x509.h
index 583bc12..2a9ce99 100644
--- a/include/mbedtls/x509.h
+++ b/include/mbedtls/x509.h
@@ -278,6 +278,26 @@
  */
 int mbedtls_x509_time_is_future( const mbedtls_x509_time *from );
 
+/**
+ * \brief          Free a dynamic linked list presentation of an X.509 name
+ *                 as returned e.g. by mbedtls_x509_crt_get_subject().
+ *
+ * \param name     The address of the first name component. This may
+ *                 be \c NULL, in which case this functions returns
+ *                 immediately.
+ */
+void mbedtls_x509_name_free( mbedtls_x509_name *name );
+
+/**
+ * \brief          Free a dynamic linked list presentation of an X.509 sequence
+ *                 as returned e.g. by mbedtls_x509_crt_get_subject_alt_name().
+ *
+ * \param seq      The address of the first sequence component. This may
+ *                 be \c NULL, in which case this functions returns
+ *                 immediately.
+ */
+void mbedtls_x509_sequence_free( mbedtls_x509_sequence *seq );
+
 #if defined(MBEDTLS_SELF_TEST)
 
 /**
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index ca93e1a..ad6140c 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -720,7 +720,8 @@
  *
  * \return          \c 0 on success. In this case, the user takes ownership
  *                  of the name context, and is responsible for freeing it
- *                  once it's no longer needed.
+ *                  through a call to mbedtls_x509_name_free() once it's no
+ *                  longer needed.
  * \return          A negative error code on failure.
  */
 int mbedtls_x509_crt_get_subject( mbedtls_x509_crt const *crt,
@@ -744,7 +745,8 @@
  *
  * \return          \c 0 on success. In this case, the user takes ownership
  *                  of the name context, and is responsible for freeing it
- *                  once it's no longer needed.
+ *                  through a call to mbedtls_x509_name_free() once it's no
+ *                  longer needed.
  * \return          A negative error code on failure.
  */
 int mbedtls_x509_crt_get_issuer( mbedtls_x509_crt const *crt,