Reported clock rate for decode is different than clock rate in config files

[tdaede]

In ibm*.textpb files, the clock rate is specified as 500kHz for HD media:

			clock_rate_khz: 500

However, when decoding HD media, the reported clock rate is shown as double:

 36.1: 290.11225 ms in 123425 bytes
       26 records, 13 sectors; 1.00us clock (998kHz); 8 distinct sectors; 
       logical track 36.1; 12288 bytes decoded.

[davidgiven]

This is happening because internally the decoder is using bits-per-second for the clock, which of course is half the MFM or FM clock rate (but correct for GCR). That should be relatively easy to correct in the appropriate decoders.