os: simplify WriteString It was working too hard.
os: simplify WriteString
It was working too hard.

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/4639081

apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,ssrl/go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,ssrl/go,tv42/old-go,gnanderson/go,dsriram/golang,ssrl/go,ssrl/go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,ssrl/go,apparentlymart/golang,gnanderson/go,ssrl/go,tv42/old-go,tv42/old-go,tv42/old-go,dsriram/golang,apparentlymart/golang,ssrl/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,gnanderson/go