[Ada] Sockets.Poll implementation

[Ada] Sockets.Poll implementation

gcc/ada/

	* Makefile.rtl (GNATRTL_SOCKETS_OBJS): New object
	g-socpol$(objext) New source files noted: g-socpol.adb,
	g-socpol.ads, g-socpol__dummy.adb, g-socpol__dummy.ads,
	g-sopowa.adb, g-sopowa__posix.adb, g-sopowa__mingw.adb,
	g-spogwa.adb, g-spogwa.ads.
	* impunit.adb (Non_Imp_File_Names_95): New base filename
	g-socpol in "GNAT Library Units" section for GNAT.Sockets.Poll
	unit.
	* libgnat/g-socket.ads, libgnat/g-socket.adb:
	(Raise_Socket_Error): Moved from body to private part of
	specification to use in GNAT.Sockets.Poll.
	* libgnat/g-socpol.ads, libgnat/g-socpol.adb: Main unit of the
	implementation.
	* libgnat/g-socpol__dummy.ads, libgnat/g-socpol__dummy.adb:
	Empty unit for the systems without sockets support.
	* libgnat/g-spogwa.ads, libgnat/g-spogwa.adb: Generic unit
	implementing sockets poll on top of select system call.
	* libgnat/g-sopowa.adb (Wait): Separate implementation for
	operation systems with poll system call support.
	* libgnat/g-sopowa__posix.adb (Wait): Separate implementation
	for POSIX select system call.
	* libgnat/g-sopowa__mingw.adb (Wait): Separate implementation
	for Windows select system call.
	* gsocket.h (_WIN32_WINNT): Increase to 0x0600 for winsock2.h to
	allow WSAPoll related definitions.
	* s-oscons-tmplt.c: Fix comment next to #endif for
	#if defined (__linux__) || defined (__ANDROID__) line.  Include
	<poll.h> for all except VxWorks and Windows.
	(SIZEOF_nfds_t): New definition.
	(SIZEOF_fd_type): New definition.
	(SIZEOF_pollfd_events): New definition.
	(POLLIN, POLLPRI, POLLOUT, POLLERR, POLLHUP, POLLNVAL): New
	definitions for VxWorks to be able to emulate poll on top of
	select in it.  Define POLLPRI as zero on Windows as it is not
	supported there.
	(Poll_Linkname): New definition, because the poll system call
	has different name in Windows and POSIX.