utils: Fix race in verify accepting unrelated signals

Signals like VerifyResult may be received from unrelated Verify
operations. To avoid races, we need to ignore any VerifyResult that
happenes before the DBus method returns.

The only way to do this race-free is to use the async version of the
VerifyStart method.

meson.build

@@ -62,6 +62,7 @@ common_cflags = cc.get_supported_arguments([
 add_project_arguments(common_cflags, language: 'c')
 
 host_system = host_machine.system()
+# NOTE: Bump gdbus-codegen min version once we can depend on 2.64!
 glib_min_version = '2.56'
 libfprint_min_version = '1.90.1'