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

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