sys_select: don't depend on sys_socket

sys_select: don't depend on sys_socket

This is so that Emacs doesn't have to drag in m4/sockpfaf.m4 etc; see
<http://lists.gnu.org/archive/html/bug-gnulib/2011-06/msg00358.html>.
This fix works on GNU and GNU-like platforms, but has not been tested
on native Windows.
* lib/sys_select.in.h: Include <sys/socket.h> only if native Windows.
* m4/sys_select_h.m4 (gl_HEADER_SYS_SELECT): Do not require
gl_HEADER_SYS_SOCKET.
* modules/sys_select (Files): Add m4/sys_socket_h.m4, for
gl_PREREQ_SYS_H_WINSOCK2.