Handle absence of socket constants

Handle absence of socket constants

AF_SNA, AF_DECnet and SOCK_RDM are not part of POSIX, and Haiku does not
supply them.
Check for their presence before using them, as already done for AF_IPX.

2010-03-21  Andreas Faerber  <andreas.faerber@web.de>

	* socket-io.c: Don't depend on AF_SNA, AF_DECnet,
	SOCK_RDM.

This commit is licensed under the MIT X11 license.

svn path=/trunk/mono/; revision=153961