Add density code for DAT-72, and notes on DAT-160.

As it turns out, the density code for DAT-160 (0x48) is the same
as for SDLT220. Since the SDLT values are already in the table,
we will leave them in place.

Thanks to Harald Schmalzbauer for confirming the DAT-72 density code.

lib/libmt/mtlib.c:
Add DAT-72 density code, and commented out DAT-160 density
code. Explain why DAT-160 is commented out. Add notes
explaining where the bpi values for these formats came from.

usr.bin/mt/mt.1:
Add DAT-72 density code, and add a note explaining that
the SDLTTapeI(110) density code (0x48) is the same as
DAT-160.

Sponsored by: Spectra Logic
MFC after: 3 weeks