arp-scan: Add network discovery mode (--discover)

[bcoles]

Add a simple network discovery mode usable with -d or --discover flags.

Discover mode discovers routable networks by scanning the first IP
address within each /24 subnet within the RFC1918 local address space.

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

Discovery scan duration is typically less than 5 minutes.

Example output:

# time ./arp-scan -d 
Interface: eth0, type: EN10MB, MAC: 00:0c:29:6c:53:7d, IPv4: 172.16.191.192
Starting arp-scan 1.9.7 with 69615 hosts (https://github.com/royhills/arp-scan)
172.16.191.1	00:50:56:c0:00:08	VMware, Inc.
192.168.75.1	00:50:56:c0:00:08	VMware, Inc.

5 packets received by filter, 0 packets dropped by kernel
Ending arp-scan 1.9.7: 69615 hosts scanned in 279.854 seconds (248.75 hosts/sec). 2 responded

real	4m39.862s
user	0m0.296s
sys	0m20.084s