os/exec: TestExtraFiles - close any leaked file descriptors
os/exec: TestExtraFiles - close any leaked file descriptors

Ensure that file descriptors have not already been leaked into our
environment - close any that are open at the start of the
TestExtraFiles test.

Also use the appropriate command for listing open files.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5574062

tv42/old-go,apparentlymart/golang,tv42/old-go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,tv42/old-go,gnanderson/go,dsriram/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,apparentlymart/golang,gnanderson/go