syscall: fix fork-exec/wait inconsistencies for Plan 9

syscall: fix fork-exec/wait inconsistencies for Plan 9

Fixes the fork-exec/wait race condition for ForkExec
as well, by making it use startProcess. This makes the
comment for StartProcess consistent as well.

Further, the passing of Waitmsg data in startProcess
and WaitProcess is protected against possible forks
from outside of ForkExec and StartProcess, which might
cause interference with the Await call.

R=rsc, rminnich, npe, ality
CC=golang-dev
https://golang.org/cl/7128059