Add the LTO-8 Type M density code (0x5d, LTO-8M) to libmt and the
mt(1) man page.

LTO-8 Type M (also known as M8) is a pristine LTO-7 cartridge
formatted in a LTO-8 drive in a new, higher density format. It
has a separate density code, and is only readable in an LTO-8
drive.

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

usr.bin/mt/mt.1:
Add the LTO-8 Type M density code to the density
table in the mt(1) man page.

MFC after: 3 days
Sponsored by: Spectra Logic