LIB-28: get mac device name over drive node

This is a prerequisite for allowing drive numbers.
don't put mb_disc_read_unportable in unix.c

We explicitely call it in the platform files as mb_disc_unix_read().
This removes some "black magic" since all other unportable functions
are also implemented in the platform files.

This is also a prerequisite to accept numbers as drive names for Mac OS X,
since we need to intercept the device name before opening it.
LIB-28: enable mac drive numbering

Since Mac doesn't have stable disc drive device names,
one can now additionally use drive numbers.
The default is "1" which stands for the first optical disc drive.
This should be the same numbering `drutil list` uses.
better documentation of functions in unix.h
Merge branch 'mac_drive_numbers'

see pull request #37 and LIB-28

Conflicts:
	src/unix.h
ChangeLog: LIB-28