syscall: fix exec_bsd.go to accompany exec_linux.go changes
exec_plan9.go too.
Those are in CL 8334044

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/9055043
syscall: fix a bug in the shuffling of file descriptors in StartProcess on Linux.

R=iant, iant, r, bradfitz
CC=golang-dev
https://codereview.appspot.com/8334044

Committer: Rob Pike <r@golang.org>