SSH Agent: Expect passphrases to be in UTF-8

The previous default was to expect passphrases to be ASCII or
rather Latin-1. It would be reasonable to expect modern keys to
use UTF-8 instead.

This is a non-breaking change if passphrases only use characters
that fall within ASCII.

Fixes #2102

tests/TestOpenSSHKey.h

@@ -37,6 +37,7 @@ private slots:
     void testDecryptRSAAES256CBC();
     void testDecryptOpenSSHAES256CTR();
     void testDecryptRSAAES256CTR();
+    void testDecryptUTF8();
 };
 
 #endif // TESTOPENSSHKEY_H