os: cache Getwd result as hint for next time

Avoids the dot-dot-based algorithm on repeated calls
when the directory hasn't changed.

R=golang-dev, iant, bradfitz
CC=golang-dev
https://codereview.appspot.com/7340043
test/run: use correct $PWD to make os.Getwd less expensive

The commands being run are 'go tool this' and 'go tool that',
and the go command will call Getwd during its init.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7336045
cmd/go: set $PWD when running commands

This makes os.Getwd inside those commands much faster.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7324055
syscall: don't make //sys lines be doc comments

Cleans up godoc and makes it consistent. (some had it, some
didn't)

This still keeps the information there, though, for people
looking at the source directly.

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/7324056