grml-network: fix nmap usage for recent nmap versions

The --max_rtt_timeout setting wants us to use 4000ms instead
of 4000:

| % nmap -sP --host_timeout 4000 --max_rtt_timeout 4000 192.168.88.1                                                     :(
| Since April 2010, the default unit for --max-rtt-timeout is seconds, so your time of "4000" is 4000 seconds. Use "4000ms" for 4000 milliseconds.
| QUITTING!

This finally reports correct online/offline status again.