Medium: tools: send_arp.libnet: reuse ARP packets (debian#701914)

This is just a reshuffle of code to have the libnet packets
reused instead of creating new ones each time a packet is to be
sent. There are no functional changes, the program should be a
bit faster and require less resources (memory in particular).
Merge pull request #247 from dmuhamedagic/sendarp

Medium: tools: send_arp.libnet: reuse ARP packets (debian#701914)