check_bgp_counters

For those that have no SNMP trap manager, this poll-based approach will still catch short BGP outages because certain counters increment or change on BGP session failure and re-establishment.

Checks against at least six counters are made, plus changes in various prefixes. There's no need to know or enumerate the IP addresses of the BGP router's peers - the code handles that automatically. There's also no need to accommodate for modified BGP update message intervals, as the code takes that into account when calculating how many changes should have occurred since the last check.

v1.1
- fixed last BGP peer state to print text rather than numeric value
v1.2
- added host names to each of the IPs listed for BGP neighbors
v1.3
- now properly ignores strings found in OIDs, useful to ignore certain BGP neighbors
v1.4
- AS numbers are now included
- does not alarm on sessions that are administratively shut down
v1.5
- allow min/max sent/received limits to be added
v1.6
- add an option to check prefix counts (received versus installed on Brocade)