gio: move Winsock error mapping to g_io_error_from_win32_error()

gio: move Winsock error mapping to g_io_error_from_win32_error()

Rather than having special code in gsocket.c, handle Winsock errors
along with other Win32 errors in gioerror.c

Also, reference g_win32_error_message() from the
g_io_error_from_win32_error() docs, and update the
g_win32_error_message() docs to clarify that it works with Winsock
error codes too.