Add #pragma once to common.h

This file is gonna be included everywhere, so having a pragma
alongside include guards ensures the best compile-time performance.
Rename common.h to dynamips_common.h.

The name is too generic.
Delete ds1620.h.

This file is not used and most of the contatns are in dev_ds1620.c.
Tell the compiler that mem_bswap32 does not require aligned data.

Now we can be compile with -O3. I'm keeping the optimization level
at -O2 because it was reported as having better packet performance.

Final fix for issue #9 - Reproducable crash
Minimize headers.
Minimize headers.
Comments.
define FD_SETSIZE for Cygwin in dynamips_common.h