inet_pton: Support for MSVC on Windows Vista or newer.

inet_pton: Support for MSVC on Windows Vista or newer.

* lib/arpa_inet.in.h (inet_pton): Also consider REPLACE_INET_PTON.
* lib/inet_pton.c (rpl_inet_pton): Use a simple wrapper if
HAVE_DECL_INET_PTON is defined.
* m4/inet_pton.m4 (gl_FUNC_INET_PTON): Invoke gl_PREREQ_SYS_H_WINSOCK2.
On platforms with <winsock2.h>, test whether inet_pton is declared in
<ws2tcpip.h>. If so, arrange to replace it.
* m4/arpa_inet_h.m4 (gl_ARPA_INET_H_DEFAULTS): Initialize
REPLACE_INET_PTON.
* modules/arpa_inet (Makefile.am): Substitute REPLACE_INET_PTON.
* modules/inet_pton (Files): Add m4/sys_socket_h.m4.
(Depends-on, configure.ac): Update condition.
* doc/posix-functions/inet_pton.texi: Mention the MSVC problem.