ok just tried the patch...
seems that now it doesn't manage the case where we put a different user for different engineid ...
now it generate a conf file withe the good "creatuser -e <engineid> lines" but use the 
same user each time.

I changed the username for one switch in switches.conf but then it keep the previous lines in the generated 
snmptrad.conf.

actually with "our" patch it works ;)

good idea to have service/snmptrapd.pm now 

snmptrapd needs "-e <engineid>" lines , without them the /usr/local/pf/log/snmptrapd.log 
keep empty , it needs it to decipher the traps .