bus: support temporarily const errors that don't need to be freed but require deep copies

This should fix issues with incorrectly copying bus error messages out
of sd_bus_message objects.

Original bug found by: Djalal Harouni
bus: use "System.Error." as prefix for implicit errno bus errors

We probably shouldn't use "Posix.Error" since many of the errors have
nothing to do with Posix.
update TODO