compress/gzip: allow stopping at end of first stream

Allows parsing some file formats that assign special
meaning to which stream data is found in.

Will do the same for compress/bzip2 once this is
reviewed and submitted.

Fixes issue 6486.

LGTM=nigeltao
R=nigeltao, dan.kortschak
CC=adg, bradfitz, golang-codereviews, r
https://codereview.appspot.com/159120044
cmd/gc: disallow call of *T method using **T variable

This brings cmd/gc in line with the spec on this question.
It might break existing code, but that code was not conformant
with the spec.

Credit to Rémy for finding the broken code.

Fixes issue 6366.

LGTM=r
R=golang-codereviews, r
CC=adonovan, golang-codereviews, gri
https://codereview.appspot.com/129550043