libctrl: Add commands to query counters and rate_cntr

These commands are installed in controlif_setup. Query them like this:
"rate_ctr.<interval>.<counter group>.<index>.<counter name>" for rate
counters and "counter.<counter name>" for regular counters. <interval>
may be either "abs" for absolute values or one or
"per_{sec,min,hour,day}".

It is possible to query all rate counters in a group (regardless of
index) or all counters in a group and with a certain index if you omit
<counter name> and <index> or just <counter name>.

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