net/textproto: avoid 1 copy in ReadLine, ReadContinuedLine
net/textproto: avoid 1 copy in ReadLine, ReadContinuedLine

Fixes issue 2083.

R=msolo, bradfitz
CC=golang-dev
http://codereview.appspot.com/4812042

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