syscall: take over env implementation

The environment is needed by package time, which
we want not to depend on os (so that os can use
time.Time), so push down into syscall.

Delete syscall.Sleep, now unnecessary.

The package os environment API is preserved;
it is only the implementation that is moving to syscall.

Delete os.Envs, which was undocumented,
uninitialized on Windows and Plan 9, and
not maintained by Setenv and Clearenv.
Code can call os.Environ instead.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5370091