Supports setting the gdb listen port
Supports setting which version of stlink to use
Supports setting the device to use (in case autoprobing doesn't work)

This will let people with Macintosh OS/X computers compile and use stlink with STLink/V2 devices, as libsg3 is not available for that platform.

Minor improvements to utilities and example makefile

Conflicts:
	src/stlink-usb.c