os: document that users of Fd should keep f alive

Fixes issue 9046.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/162680043
runtime: fix initial gp->sched.pc in newextram

CL 170720043 missed this one when adding +PCQuantum.

LGTM=iant
R=r, iant
CC=golang-codereviews
https://codereview.appspot.com/168090043