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/Console.cpp

@@ -83,7 +83,7 @@ Kernel::KResultOr<size_t> Console::write(Kernel::FileDescription&, size_t, const
         return (ssize_t)bytes_count;
     });
     if (nread < 0)
-        return Kernel::KResult(nread);
+        return Kernel::KResult((ErrnoCode)-nread);
     return (size_t)nread;
 }