Expose lf_load_keyfile outside sshpubk.c.

I'm about to use it in cmdgen for a minor UI improvement. Also, I
expect it to be useful in the Pageant client code sooner or later.

While I'm here, I've also tweaked its UI a little so that it reports a
more precise error, and provided a version that can read from an
already open stdio stream.

defs.h

@@ -64,6 +64,7 @@ typedef struct FontSpec FontSpec;
 typedef struct bufchain_tag bufchain;
 
 typedef struct strbuf strbuf;
+typedef struct LoadedFile LoadedFile;
 
 typedef struct RSAKey RSAKey;