syscall: fix a number of exec bugs on Plan 9

syscall: fix a number of exec bugs on Plan 9

1. Readdirnames was erroneously returning an
   empty slice on every invocation.

2. The logic for determining which files to
   close before exec was incorrect.  If the
   set of files to be kept open (provided by
   the caller) did not include the files
   opened at startup, those files would be
   accidentally closed.

I also cleaned up readdupdevice while I was
in the vicinity.

R=golang-dev, seed, rsc
CC=golang-dev
https://golang.org/cl/6016044