dup3: don't set FD_CLOEXEC on failure on kernels without dup3 syscall

dup3: don't set FD_CLOEXEC on failure on kernels without dup3 syscall

this is the best-effort fallback path for kernels that can't actually
support the dup3 functionality. it was setting FD_CLOEXEC flag on the
target fd (new) even if the dup2 operation failed. normally that
shouldn't happen under correct usage, but it's possible if the source
fd is not open or intentionally invalid (e.g. -1).