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

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