runtime: refactor chan code

runtime: refactor chan code
1. Make internal chan functions static.
2. Move selgen local variable instead of a member of G struct.
3. Change "bool *pres/selected" parameter of chansend/chanrecv to "bool block",
   which is simpler, faster and less code.
-37 lines total.

LGTM=rsc
R=golang-codereviews, dave, gobot, rsc
CC=bradfitz, golang-codereviews, iant, khr
https://golang.org/cl/58610043