Support $! stringification of socket error codes on Windows.

Support $! stringification of socket error codes on Windows.

The winsock error codes from WSAGetLastError() are stored by
Perl in errno, and there is some code in win32_strerror() that
would stringify them, but that code is never called when Perl
is built with the default WIN32IO_IS_STDIO setting.

This patch enables the win32_strerror() override unconditionally
and also fixes a potential memory corruption issue by using
the FORMAT_MESSAGE_IGNORE_INSERTS flag to ignore any parameter
substitution codes that may be embedded in the error message.

This now works as expected:

C:\git\perl>perl -Ilib -MPOSIX -E "$!=POSIX::EWOULDBLOCK; say $!"
A non-blocking socket operation could not be completed immediately.