sys_select: Fix compilation error on mingw, introduced on 2011-06-30.

sys_select: Fix compilation error on mingw, introduced on 2011-06-30.

* lib/sys_select.in.h: Don't include <sys/socket.h>. Instead, include
<winsock2.h>.
(rpl_fd_isset, FD_ISSET): New definitions, copied from
lib/sys_socket.in.h.
(close, gethostname): Hide declarations from <winsock2.h>.
(socket, connect, accept, bind, getpeername, getsockname, getsockopt,
listen, recv, send, recvfrom, sendto, setsockopt, shutdown): Likewise.
* lib/sys_socket.in.h (close, gethostname): Tweak indentation.
(select): Don't override if gnulib's <sys/select.h> was already
included.
* lib/unistd.in.h (socket, connect, accept, bind, getpeername,
getsockname, getsockopt, listen, recv, send, recvfrom, sendto,
setsockopt, shutdown, select): Tweak indentation.