src/udev/udev-ctrl.c: restore accept4_fallback code

This code was accidentally removed in commit

    601c4e8ce69338652b3c9178386cbc05de8bae8d

We restore it here.  Since most modern systems have accept4()
this code will seldom be triggered.  You can test by configuring
as follows:

    ac_cv_have_decl_accept4="no" ./configure

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>