certificate: Return signature scheme and parameters from issued_by() method

This also required some include restructuring (avoid including library.h
in headers) to avoid unresolvable circular dependencies.

src/libtls/tls_prf.c

@@ -17,6 +17,8 @@
 
 typedef struct private_tls_prf12_t private_tls_prf12_t;
 
+#include <library.h>
+
 /**
  * Private data of an tls_prf_t object.
  */