ask-password: add Varlink API for querying passwords from the user

This turns systemd-ask-password into a small Varlink service, so that
there's an standard IPC way to ask for a password. It mostly directly
exposes the functionality of the Varlink service.

src/test/test-varlink-idl.c

@@ -11,6 +11,7 @@
 #include "varlink-idl-util.h"
 #include "varlink-io.systemd.h"
 #include "varlink-io.systemd.BootControl.h"
+#include "varlink-io.systemd.AskPassword.h"
 #include "varlink-io.systemd.Credentials.h"
 #include "varlink-io.systemd.Import.h"
 #include "varlink-io.systemd.Journal.h"
@@ -194,6 +195,8 @@ TEST(parse_format) {
         print_separator();
         test_parse_format_one(&vl_interface_io_systemd_MachineImage);
         print_separator();
+        test_parse_format_one(&vl_interface_io_systemd_AskPassword);
+        print_separator();
         test_parse_format_one(&vl_interface_xyz_test);
 }