nrpe: switch from xinetd to nrpe.service
    
    Updating hosts to 15.5 move /etc/xinetd.d/nrpe to *.rpmsave.
    Instead of fixing this with another highstate round to bring the file
    back, switch to using nrpe.service and drop the xinetd config file.
    
    Also allow access from monitor.i.o.o in nrpe.cfg (previously, 127.0.0.1
    was enough because xinetd did forward the connection).