syscall: remove more NxM assumptions on Plan 9
syscall: remove more NxM assumptions on Plan 9

LGTM=0intro, r
R=0intro, r
CC=ality, dave, golang-codereviews, jas, mischief, rsc
https://codereview.appspot.com/111910043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go