Update check_snmp.c

We have some OIDs on our network that only report in bps (Estimated Bandwidth/Ethernet Bandwidth for some Radwin devices, etc.).

I'd like an easy way to convert from bps to Mbps, so I've changed the code so that in the case of a specific -u opt (unitv), such as "bps->Mbps" it does the math, changes the label, and reports as specified.

The double reports precision to 4 decimal places no matter which rate you're converting to.

I hope this can help someone else.