MS-DOS compatibility

[jwt27]

This small patch will make asio compile for 32-bit DOS (djgpp), with the Watt32
socket library.

Some IPV6 option macros are missing, I don't know exactly how complete the
ipv6 implementation is. Defining those to 0 gets it to compile, but using them
will of course do nothing.

I tried some of the example programs. They compiled with no warnings, and seem
to run fine.

[jwt27]

I did discover a bug in libc that prevents ioctl() arguments from being
passed through to the socket library. So the workaround is to use
ioctlsocket(), but it takes a char*, so requires extra casts.

I sent in a patch to the mailing list. If that gets picked up soon then I
might drop the last commit, since it just adds extra clutter.

This also uncovered a bug/quirk in the way the socket library handles a
non-blocking connect(). Working on a PR for that now.

[jwt27]

Better solution: ioctlsocket() now takes a void* argument, so there's no need for extra casts (ref: gvanem/Watt-32#81).

The issue with non-blocking connect() also got fixed (ref: gvanem/Watt-32#80).