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

bryanxu/go-zh,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,webfd/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,webfd/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,Triskite/willstone-goclone,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,rflanagan/reginaldflanagan-project1,d0f/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,mhennings/marcohennings-go,Triskite/willstone-goclone,bryanxu/go-zh,bryanxu/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,Triskite/willstone-goclone,d0f/go-zh,Triskite/willstone-goclone,mhennings/marcohennings-go,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,Triskite/willstone-goclone,mhennings/marcohennings-go,bryanxu/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,mhennings/marcohennings-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,d0f/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,d0f/go-zh,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof