Cleanup and simplify winsock split changes.

Cleanup and simplify winsock split changes.
Simplification.
* lib/sys_socket.in.h (_gl_close_fd_maybe_socket): Remove declaration.
* lib/close.c (_gl_close_fd_maybe_socket): Make static.

Simplification.
* lib/ioctl.c (ioctl): Don't undefine.
* lib/socket.c (socket): Don't undefine.

Remove unused module indicator macros.
* m4/sys_socket_h.m4 (gl_SYS_SOCKET_MODULE_INDICATOR): Don't define
GNULIB_$1 as a C macro.

* doc/posix-functions/close.texi: Undo last change.
* doc/posix-functions/ioctl.texi: Merge the two paragraphs about
Windows platforms.