Astribank Metering Interface
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In analog phone lines, some PBX'es can generate/detect special tones
used for billing the call (phone metering).

Astribank FXS and FXO drivers now support these features if compiled
with -DWITH_METERING

Software interface:
  FXS - To generate a metering pulse, write the channel number (0-7) into
        the file:

		/proc/xpp/XBUS-nn/XPD-mm/metering_gen

	Where:
		nn = XBUS number
		mm = XPD number

	Writing channel number 31 signifies a broadcast  -- i.e: generate
	metering pulse on all channels.

	Example:

		echo 31 > /proc/xpp/XBUS-00/XPD-00/metering_gen

  FXO - To read current metering counters from an FXO, simply read:

		/proc/xpp/XBUS-nn/XPD-mm/metering_read

	The format is self documenting:

		# Chan  Meter (since last read)
		0       5
		1       0
		2       12
		...

	Reading this file reset the counters. So the numbers always show
	the meters since last read.

	The metering register should be sampled at a rate which is double
	than the highest expected metering pulses. This rate is adjustable
	via a module parameter "poll_metering_interval". The current default
	is 500ms which is good if the metering pulses are sent not more
	often than once per second.

Remark: The FXS hardware can generate maximum 0.85 volt RMS, while the FXO
        needs a minimum of 1.1 vold RMS for detection. Therefore, this
	feature cannot be tested in a loopback (Astribank-FXS to Astribank-FXO).