[Click] Update to FromSimDevice

[Ruben Merz]

Dear all,

In the patch pushed recently to support Click on ns-3, I did not pay 
attention to the fact that I used mandatory arguments (used read_mp 
instead of read_p). In particular, the patch made the argument SNAPLEN 
mandatory.

The three patches in 
https://github.com/rmerz/click/commits/ruben/ns3click address this 
issue, but also make the argument order of FromSimDevice compatible with 
FromDevice.u. Doing so means that the ns-2 patches have to be slightly 
modified (addressed too in the patch series). Finally, the documentation 
is slightly updated

@Eddie: I sent you a pull request

Thanks
Ruben