Make HLE disposable safely (#850)

* Make HLE disposable safely

This fix the oldest issue with the HLE code: the kernel side
disposability.

Changelog:

- Implement KProcess::UnpauseAndTerminateAllThreadsExcept, KThread::Terminate, KThread::TerminateCurrentProcess, KThread::PrepareForTermiation and the svc post handler accurately.
- Implement svcTerminateProcess and svcExitProcess. (both untested)
- Fix KHandleTable::Destroy not decrementing refcount of all objects stored in the table.
- Spawn a custom KProcess with the maximum priority to terminate every guest KProcess. (terminating kernel emulation safely)
- General system stability improvements to enhance the user's experience.

* Fix a typo in a comment in KProcess.cs

* Address gdk's comments

Ryujinx.HLE/HOS/Kernel/Process/KHandleTable.cs

@@ -272,6 +272,7 @@ namespace Ryujinx.HLE.HOS.Kernel.Process
                             disposableObj.Dispose();
                         }
 
+                        entry.Obj.DecrementReferenceCount();
                         entry.Obj  = null;
                         entry.Next = _nextFreeEntry;