Permit system calls to be designated as non-blocking, meaning
that we simply call them without involving the scheduler.
This change by itself is mostly performance neutral. In
combination with a following change to the net package there
is a performance advantage.
R=rsc, dfc, r2, iant2, rsc1
CC=golang-dev
https://golang.org/cl/4278055
because they are in package runtime.
another step to enforcing package boundaries.
R=r
DELTA=732 (114 added, 93 deleted, 525 changed)
OCL=35811
CL=35824