monitor: Print channel latency information with -a/--analyze

monitor: Print channel latency information with -a/--analyze

This prints channel latency information and use gnuplot when using
-a/--analyze to plot a graph of channels:

  Found TX L2CAP channel with CID 68
        PSM 25
        22490 packets
        7-50 msec (~11 msec) TX Latency
        850-850 octets TX packet size
        ~569 Kb/s TX transfer speed

  10000 +-+----------------------------------------------------------------+
         +|     ++                                                         |
         +|     || +                                       Packets +-----+ |
         +|   + || |                                                       |
   1000 +-|   | || |+ ++                                                   |
         +|   | || || ||   +                                               |
         +|   | || || ||+ +|                                               |
         +|   | || || ||| || ++ ++                                         |
    100 +-|   | || || ||| || || ||   +                                     |
         +|   | || || ||| || || || ++| +   +                               |
         +|   | || || ||| || || || ||| |   |                               |
         +|   | || || ||| || || || ||| |+ +| +                             |
          |   | || || ||| || || || ||| || || |+ +                          |
     10 +-|   | || || ||| || || || ||| || || || |+                         |
         +|   | || || ||| || || || ||| || || || ||     +                   |
         +|  +| || || ||| || || || ||| || || || ||+ ++ |+ ++ +             |
          |  || || || ||| || || || ||| || || || ||| || || || |             |
      1 +-|  || || || ||| || || || ||| || || || ||| || || || |   +     +   |
         ++----------------------------------------------------------------+
          +      +      +       +      +      +      +       +      +      +
          5      10     15      20     25     30     35      40     45     50
                                    Latency (ms)