Add LTO-8 density codes.

lib/libmt/mtlib.c:
Add the LTO-8 density code to the density table in libmt.

usr.bin/mt/mt.1:
Add the LTO-8 density code, tracks, bpmm, and bpi to the density
table in the mt(1) man page.

MFC after: 3 days
Sponsored by: Spectra Logic