dbus-spawn: do not forget the exec() errno when the grandchild exits

dbus-spawn: do not forget the exec() errno when the grandchild exits

As is already noted in a comment in
_dbus_babysitter_set_child_exit_error(), if the grandchild fails
to exec() the desired process, we get both CHILD_EXEC_FAILED (with
an errno) and CHILD_EXITED (with a status), and we want to report
the former, since it is more informative. However, clearing
sitter->errnum meant we lose the errno value.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=24821
Reviewed-by: Ross Lagerwall