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
The callback mechanism has been made more flexible.
Eliminated one round of argument copying in Syscall.
Faster Get/SetLastError implemented.
Added gettime for gc perf profiling.
R=rsc, brainman, mattn, rog
CC=golang-dev
https://golang.org/cl/4058046
