main.lua: Add command-line argument for debugging purposes.

  * -dm = Enable developer mode. Enables debug prints and asserts.

  * This sets to true a global variable _G.developer_debug that can be
    used on the rest of snabswitch to enable prints, asserts or other
    things that are not intended for production.

  * Make all other parts of snabswitch where a debug variable was used
    (or should be used) to depend on this one.
Only print the last_size when debug is enabled (./snabbswitch -dm).

 * This code starts triggering on a kernel >= 3.14 (Ubuntu 14.04 uses
   3.13, so this went unnoticed). It causes a lot of flood because it
   triggers constantly.

 * Example of the output from snabswitch when running same_vlan.ports
   with both VMs running Kernel 3.14.23 and running an iperf test
   (MTU 1500) between both VMs:

      size=  2036
      size=  1524
      size=  2036
      size=  1524
      size=  2036
      size=  1524
      [ .... repeated ad-infinitum ]

 * I have bisected the Linux kernel, the commit that causes this print to
   trigger is fb51879dbceab9c40a39018d5322451691909e15 (merged on 3.14)
   https://git.kernel.org/linus/fb51879
Merge pull request #324 from clopez/last_size-check-debug

Add command-line argument for debugging purposes (-dm) and only print the last_size info when debug is enabled