hx509: Make hxtool default store type to FILE

A common complaint about hxtool(1) is that if one fails to add a TYPE:
prefix to a CSR or certificate/private key store names, then hxtool
fails somewhat inscrutably.  We can't just fix hx509_certs_init() or
hx509_certs_append() because they default to "MEMORY" so who knows what
might break.  Instead we fix all uses of user-provided store names in
hxtool to have a FILE: prefix if no type was given.  For CSRs we'll
default to adding the only type prefix supported, "PKCS10".

lib/hx509/keyset.c

@@ -66,7 +66,7 @@ struct hx509_certs_data {
     int flags;
 };
 
-static struct hx509_keyset_ops *
+struct hx509_keyset_ops *
 _hx509_ks_type(hx509_context context, const char *type)
 {
     int i;