gkrellm network settings code can lead to reproducable instant X server crashes

[Fionn]

A user that uses gkrellm alonside virtualbox for some months will experience problems that may crash X instantly and reproducably.

Steps to reproduce:
 * Use gkrellm with network monitors
 * Use virtualbox for some time.
   It will create virtual network devices with names that constantly change. Apparently
   gkrellm tracks all of them in its config file and never forgets any of them.
   After some time you will have a gkrellm user-config filled with hundrets of lines like this:
     net enables veth52730d0 1 1 0
     net enables veth56844d4 1 1 0
     net enables veth59140bf 1 1 0
     net chart_config veth59140bf 40 300 0 1 0 0 : 0 0 0 0 500 : 0 0 0 0 500
     net enables vethfc4e0e4 1 1 0
     net chart_config vethfc4e0e4 40 5 0 1 0 0 : 0 0 0 0 500 : 0 0 0 0 500
     net enables vethf11a8dd 1 1 0
     net enables vethf94ed79 1 1 0
     net enables vethf620cbd 1 1 0
     net enables vethfeca6a1 1 1 0
     net enables veth59748a4 1 1 0
     net chart_config veth59748a4 40 300 0 1 0 0 : 0 0 0 0 500 : 0 0 0 0 500
     net enables veth2214351 1 1 0
 * now try to open the network monitor config panel in gkrellm. Your X will crash instantly.