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/FileSystem/File.cpp

@@ -61,7 +61,7 @@ int File::ioctl(FileDescription&, unsigned, FlatPtr)
 
 KResultOr<Region*> File::mmap(Process&, FileDescription&, VirtualAddress, size_t, size_t, int, bool)
 {
-    return KResult(-ENODEV);
+    return ENODEV;
 }
 
 }