net: break up and simplify Dial a bit
net: break up and simplify Dial a bit

No API or functional change.

R=jacek.masiulaniec, mikioh.mikioh, rsc
CC=golang-dev
http://codereview.appspot.com/4630081

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