Linux/macOS: surface post-sudo elevation errors

Separate sudo authentication success from elevated request execution state by acknowledging when the elevated core service starts successfully.

Once that channel is available, propagate later failures instead of showing repeated administrator password dialogs. Initialize the sudo dummy-password flag consistently on macOS, avoiding an uninitialized read of UseDummySudoPassword.

Register admin-password cleanup before elevation attempts so plaintext sudo credentials are erased on early-return and post-sudo failure paths.

References #1788.

src/Core/CoreBase.cpp

@@ -20,9 +20,7 @@ namespace VeraCrypt
 {
 	CoreBase::CoreBase ()
 		: DeviceChangeInProgress (false)
-#if defined(TC_LINUX ) || defined (TC_FREEBSD)
 		, UseDummySudoPassword (false)
-#endif
 #if defined(TC_UNIX)
 		,AllowInsecureMount (false)
 #endif