Add link3dsDisconnectFromHost()

[ZeroSkill1]

This fixes a potential crash when linking stdout/stderr to 3dslink output.

Even when closing the socket as instructed in the docstring, a crash can occur.

In socExit, the soc device is removed from devoptab_list, which invalidates the device in the file descriptor for the socket/stderr/stdout. During an exit routine, newlib calls into cleanup_stdio, which closes the std streams.
This creates the case where handles[STD_OUT] and handles[STD_ERR] are valid but the corresponding devoptab entry is no longer alive. Hence, close() on stdio/err causes a segfault.

The solution is to close both stdout and stderr along with the socket when disconnecting from the 3dslink host. For convenience, this has been placed into a wrapper, link3dsDisconnectFromHost.