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/req.c

@@ -927,7 +927,8 @@ hx509_request_parse(hx509_context context,
     /* XXX Add support for PEM */
     if (strncmp(csr, "PKCS10:", 7) != 0) {
 	hx509_set_error_string(context, 0, HX509_UNSUPPORTED_OPERATION,
-			       "unsupport type in %s", csr);
+                               "CSR location does not start with \"PKCS10:\": %s",
+                               csr);
 	return HX509_UNSUPPORTED_OPERATION;
     }