PK: add nice interface functions

Also fix a const-corectness issue.
diff --git a/include/polarssl/error.h b/include/polarssl/error.h
index 45a6640..10e68f8 100644
--- a/include/polarssl/error.h
+++ b/include/polarssl/error.h
@@ -84,7 +84,7 @@
  * ECP      4   4 (Started from top)
  * MD       5   4
  * CIPHER   6   5
- * SSL      6   5 (Started from top)
+ * SSL      6   6 (Started from top)
  * SSL      7   31
  *
  * Module dependent error code (5 bits 0x.08.-0x.F8.)