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/version-script.map

@@ -17,6 +17,7 @@ HEIMDAL_X509_1.2 {
 		_hx509_generate_private_key_init;
 		_hx509_generate_private_key_is_ca;
 		_hx509_get_cert;
+		_hx509_ks_type;
 		_hx509_make_pkinit_san;
 		_hx509_map_file_os;
 		_hx509_name_from_Name;