net/http: make strace test more robust
net/http: make strace test more robust

Apparently some distros don't let you ptrace attach
to your own existing processes.

Run strace on the child directly, instead, which
reportedly is more often allowed, and makes the
code simpler too.

R=golang-dev, n13m3y3r
CC=golang-dev
http://codereview.appspot.com/5675050

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