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

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang