os/exec: tell lsof not to block

os/exec: tell lsof not to block

For some reason lsof is now hanging on my workstation
without the -b (avoid blocking in the kernel) option.
Adding -b makes the test pass and shouldn't hurt.

I don't know how recent the -b option is.  If the builders
are ok with it, it's probably ok.

LGTM=rsc
R=golang-codereviews, bradfitz, rsc
CC=golang-codereviews
https://golang.org/cl/166220043