Simplify and cleanup the current SNMP data mapping

[aquette]

SNMP data mapping (SNMP OIDs to NUT variables) are currently hardcoded in *-mib.c files.
The next generation of SNMP driver ( #20 ) will use external data mapping (i.e., not hardcoded but text files).
However, most *-mib.c files use #define and other things that don't ease the current maintenance nor evolution, and that won't ease future conversion.
This should be limited to the following list of files:

• apcc-mib.c
• compaq-mib.c
• eaton-mib.c
• ietf-mib.c
• netvision-mib.c
• powerware-mib.c

Moreover, some structures (such as value lookup structures, ex: outlet_status_info) can be colliding, and should be added a suitable suffix (ex: outlet_status_info => eaton_marlin_outlet_status_info)

Todo list

• have only 1 mib2nut per implementation (.c) file
• have the implementation (.c and .h) files named after the mib2nut structure (i.e. "x.c / x.h => mib2nut x = ...")
• replace all defines in structure by their values
• have unique name for all value lookup structures