Tweaks and additions for building under OSX 10.8.3

I added a new network mode for OS X called "etherslave" that allows uses the bpf interface to read and write raw packets from an ethernet interface.  This, among other things, allows Appletalk to work with another machine on the same LAN.

There seems be an issue with 16-bit video in the gtk video build under OS X, which also affects 8 bit, 4 bit and 2-bit video.  As a work around I changed the order that local video modes are searched, so that (for example) 8-bit video is mapped to local 24-bit instead of local 16-bit video.  This fixed 8-bit, 4-bit and 2-bit video.

The mac os gui code does not build under the latest OS X, however the sound code does.  Therefore, I separated out the sound into its own configuration option so you can build with gdk video and mac os sound.

I configure with the following options in the Unix directory under OS X 10.8.3:

./autogen.sh --enable-macosx-sound  CC=clang CXX=clang  --enable-macosx-etherslave