Squashed 'contrib/signify/' content from commit a1554a4

git-subtree-dir: contrib/signify
git-subtree-split: a1554a4384c794c004da70bd006ce93101f77426

util.h

@@ -0,0 +1,5 @@
+#include <stdint.h>
+
+int bcrypt_pbkdf(const char *pass, size_t pass_len, const uint8_t *salt,
+    size_t salt_len, uint8_t *key, size_t key_len, unsigned int rounds);
+void explicit_bzero(void *b, size_t len);