Kernel+LibC: Turn errno codes into a strongly typed enum

..and allow implicit creation of KResult and KResultOr from ErrnoCode.
This means that kernel functions that return those types can finally
do "return EINVAL;" and it will just work.

There's a handful of functions that still deal with signed integers
that should be converted to return KResults.

Kernel/Devices/KeyboardDevice.cpp

@@ -386,7 +386,7 @@ KResultOr<size_t> KeyboardDevice::read(FileDescription&, size_t, UserOrKernelBuf
             return (ssize_t)data_bytes;
         });
         if (n < 0)
-            return KResult(n);
+            return KResult((ErrnoCode)-n);
         ASSERT((size_t)n == sizeof(Event));
         nread += sizeof(Event);