Implement SNMP-index conversion rules.

SNMP-index conversion rules similar to fproble-ulog, as suggested and
submitted in
  https://sourceforge.net/p/ipt-netflow/bugs-requests-patches/58/

Quoting man fprobe-ulog:

   Comma separated list of interface name to SNMP-index conversion
  rules.  Each rule consists of interface base name and SNMP-index
  base separated by colon (e.g. ppp:200). Final SNMP-index is  sum
  of corresponding SNMP-index base and interface number.
  In the above example SNMP-index of interface ppp11 is 211.

   If interface  name  did not fit to any of conversion rules then
  SNMP-index will be taken from kernel.

This implementation isn't optimized for performance (no rule caching
or hashing), but maybe it's fast anyway (tried to make it cpu cache
friendly). So fewer rules are better.

Rules are parsed in order from first to last until first match.

Thanks to Valentin V. Yankin (prototype implementation) and Alexander
Demenshin (for convincing me).