Prevent constant memory usage growth in passive mode

[hit0ri]

Since pcap handle only gets closed when program exits, memory consumption is constantly growing. It is particularly noticeable when find3-cli-scanner is executed with --forewer. In my tests memory usage grows from 18-20 MB to 90-105 MB in 10 minutes.

This commit attempts to prevent this by closing pcap handle on scanSeconds, thus freeing up memory.

pprof top output after 10 minutes of runtime in passive mode (before):

$ go tool pprof -top -base base.heap ./find3-cli-scanner 10m.heap
File: find3-cli-scanner
Build ID: 646efc6af6afa1e2cd0809ed006e96f00c4553cf
Type: inuse_space
Time: Sep 8, 2018 at 4:02pm (EEST)
Showing nodes accounting for 28165.31kB, 100% of 28165.31kB total
      flat  flat%   sum%        cum   cum%
16385.75kB 58.18% 58.18% 18434.50kB 65.45%  github.com/google/gopacket/layers.decodeDot11InformationElement
 6658.38kB 23.64% 81.82% 11779.03kB 41.82%  github.com/google/gopacket.NewPacket
 2048.75kB  7.27% 89.09%  2048.75kB  7.27%  github.com/google/gopacket.(*packet).AddLayer
 1536.28kB  5.45% 94.55%  5632.70kB 20.00%  github.com/google/gopacket/layers.decodeDot11
 1024.12kB  3.64% 98.18%  5632.71kB 20.00%  github.com/google/gopacket/layers.decodeRadioTap
  512.03kB  1.82%   100%  4608.47kB 16.36%  github.com/google/gopacket/layers.decodeDot11MgmtBeacon
         0     0%   100% 20482.81kB 72.72%  github.com/google/gopacket.(*LayerType).Decode
         0     0%   100% 11779.03kB 41.82%  github.com/google/gopacket.(*PacketSource).NextPacket
         0     0%   100% 11779.03kB 41.82%  github.com/google/gopacket.(*PacketSource).packetsToChannel
         0     0%   100% 20482.81kB 72.72%  github.com/google/gopacket.(*eagerPacket).NextDecoder
         0     0%   100%  5120.66kB 18.18%  github.com/google/gopacket.(*eagerPacket).initialDecode
         0     0%   100% 21506.94kB 76.36%  github.com/google/gopacket.DecodeFunc.Decode
         0     0%   100% 20482.81kB 72.72%  github.com/google/gopacket.LayerType.Decode
         0     0%   100%  5120.66kB 18.18%  github.com/google/gopacket/layers.(*LinkType).Decode
         0     0%   100%  5632.71kB 20.00%  github.com/google/gopacket/layers.LinkType.Decode
         0     0%   100% 20482.81kB 72.72%  github.com/google/gopacket/layers.decodingLayerDecoder

pprof top output after 10 minutes of runtime in passive mode (after):

$ go tool pprof -top -base base.heap ./find3-cli-scanner 10m.heap
File: find3-cli-scanner
Build ID: 6fb26d1113f2679e81891dd4dfde5910c5341470
Type: inuse_space
Time: Sep 8, 2018 at 4:15pm (EEST)
Showing nodes accounting for 0, 0% of 0 total
      flat  flat%   sum%        cum   cum%

[schollz]

Cool thanks!