Add LXD edfa/emtx devices

In a network we monitor there are Langxunda (Lxd) Externally Modulated Optic Transmitters and Erbium Dotted Fibre Amplifiers. E.g:

http://www.langxunda.com/en/goods/1167.html

The current OS detection for the NSCRTV-ROOT mib does not work on them. So I added rules for them in os.inc.php.

The limits which are set for the sensors in nscrtv-root.inc.php do not work for us. So I added some checks to not set a limit. Additionally I added some more sensors.

We do not own the copyright of the Lxd logo, I don't know what your policy is, wrt the use of logos. I got it from the web page and cropped it to look nicer.

It would be nice if the attached patch and logo could be added. If there are changes needed, please let me know (style, naming convention, etc).