Unix Pageant: -E option to load key files encrypted.

This applies to both server modes ('pageant -E key.ppk [lifetime]')
and client mode ('pageant -a -E key.ppk').

I'm not completely confident that the CLI syntax is actually right
yet, but for the moment, it's enough that it _exists_. Now I don't
have to test the encrypted-key loading via manually mocked-up agent
requests.

pageant.h

@@ -220,7 +220,7 @@ enum {
     PAGEANT_ACTION_NEED_PP   /* need passphrase: *retstr is key comment */
 };
 int pageant_add_keyfile(Filename *filename, const char *passphrase,
-                        char **retstr);
+                        char **retstr, bool add_encrypted);
 void pageant_forget_passphrases(void);
 
 struct pageant_pubkey {