[Ada] New routine GNAT.Sockets.Create_Socket_Pair

[Ada] New routine GNAT.Sockets.Create_Socket_Pair

New routine to create 2 connected sockets. This routine is analog of the
UNIX system call socketpair. On UNIX platforms it is implemented on the
base of socketpair. On other platforms it is implemented by conecting
network sockets over loopback interface.

2019-09-19  Dmitriy Anisimkov  <anisimko@adacore.com>

gcc/ada/

	* libgnat/g-socket.ads, libgnat/g-socket.adb
	(Create_Socket_Pair): New routine.
	* libgnat/g-socthi.ads (OS_Has_Socketpair): Boolean constant.
	(C_Socketpair): New imported routine.
	* libgnat/g-socthi__mingw.ads, libgnat/g-socthi__vxworks.ads
	(Default_Socket_Pair_Family): New constant.
	(C_Socketpair): New routine.
	* libgnat/g-socthi__mingw.adb, libgnat/g-socthi__vxworks.adb
	(C_Socketpair): Is separated in anouther file.
	* libgnat/g-sthcso.adb (C_Socketpair): Non UNIX implementation.
	* libgnat/g-stsifd__sockets.adb: Reuse C_Socketpair.

From-SVN: r275951