Close stdin/out after child is forked

Close stdin/out after child is forked

Testcase:

{ sleep 1; yes | head -n99999; } | strace -o/dev/null sh -c 'exec <&-; sleep 9'

The writer (head) will be able to perform writes to write end of the pipe.
With strace call removed, head will immediately get SIGPIPE.

This change fixes this: now writer immediately gets SIGPIPE with strace too.

* strace.c (startup_child): Close stdin/out and reopen them to /dev/null.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>