the data chunk is the final one, which makes that zlib issues the
proper termination marker
(KAME IPSEC does this, but doesn't check eagerly in the receive
path, so the missing termination didn't cause problems so far)
closes my PR kern/44539
being here, replace the Z_PARTIAL_FLUSH flag which is marked
deprecated by zlib by Z_SYNC_FLUSH in the decompression path
(tested with IPv4 IPCOMP on i386)