Add the LTO-9 density code to libmt and the mt(1) man page.

These values are taken directly from the density report from an
IBM LTO-9 tape drive. (Using mt getdensity)

A LTO-9 drive stores 18TB raw (45TB with compression) on an LTO-9 tape.

lib/libmt/mtlib.c:

Add the LTO-9 density code, and bpmm/bpi values.

usr.bin/mt/mt.1:

Add the LTO-9 density code, bpmm/bpi values and number of

tracks. Bump the man page date.

MFC after: 3 days
Sponsored by: Spectra Logic