implement tolerant gz header parser

implement tolerant gz header parser

Since they assume a stream object, we cannot rely on the original
tarfile GZ handling. Add a “tolerant” one according to the format
spec that notices malformed or unexpected (in Deltatar context)
values, but glosses over them if they do not necessarily impact
the readability of the object.

Also use the new symbolic constants in the existing GZ reader
instead of magic numbers.