socklen: do not depend on sys_socket

socklen: do not depend on sys_socket

While trying to modify Emacs to use gnulib's socklen module,
I discovered a circular dependency: socklen depends on sys_socket
and vice versa.  Emacs can use socklen, but it does not need
sys_socket because it has its own substitute for sys/socket.h.
* m4/socklen.m4 (gl_SOCKET_HEADERS): New macro, extracted from
gl_TYPE_SOCKLEN_T.
(gl_CHECK_SOCKET_HEADERS): New macro, taken from parts of
gl_PREREQ_SYS_H_SOCKET.
(gl_TYPE_SOCKLEN_T): Require it instead of requiring
gl_PREREQ_SYS_H_SOCKET.
* m4/sys_socket_h.m4 (gl_PREREQ_SYS_H_SOCKET): Require
gl_CHECK_SOCKET_HEADERS instead of doing its work ourselves.
* modules/socklen (Depends-on): Do not depend on sys_socket.
(Include): Adjust to match the code used in gl_SOCKET_HEADERS.