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/library/x509.c b/library/x509.c
index 72cadd0..55726da 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -1194,6 +1194,28 @@
 }
 #endif /* MBEDTLS_HAVE_TIME_DATE */
 
+void mbedtls_x509_name_free( mbedtls_x509_name *name )
+{
+    while( name != NULL )
+    {
+        mbedtls_x509_name *next = name->next;
+        mbedtls_platform_zeroize( name, sizeof( *name ) );
+        mbedtls_free( name );
+        name = next;
+    }
+}
+
+void mbedtls_x509_sequence_free( mbedtls_x509_sequence *seq )
+{
+    while( seq != NULL )
+    {
+        mbedtls_x509_sequence *next = seq->next;
+        mbedtls_platform_zeroize( seq, sizeof( *seq ) );
+        mbedtls_free( seq );
+        seq = next;
+    }
+}
+
 #if defined(MBEDTLS_SELF_TEST)
 
 #include "mbedtls/x509_crt.h"