[Xymon] More patches for SNMP processing

Hi List,

Having now completed my investigation into SNMP handling in Xymon, I have some more changes to propose.? Basically, this change introduces an extra method for finding indexes in SNMP tables when nothing that is provided is unique.

I've also allowed for negative values and decimal points in returned data since both seemed to happen very quickly for me (no, the negative values weren't an error - they were wireless signal strengths).

I have some documentation updates in progress to try and fill some of the holes in this area. I send them soon.

Cheers,

Brian Scott