If stdin/stdout aren't open on startup, open them to /dev/null

If stdin/stdout aren't open on startup, open them to /dev/null

Otherwise, -oLOGFILE may end up using one of them. This conflicts
with the previous change, which closes stdin/out in strace.

* strace.c (init): If fcntl(F_GETFD) fails on stdin or stdout,
sanitize them: open /dev/null and dup it until fds 0,1,2
are all used up.

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