Support long URLs in w32-shell-execute

Support long URLs in w32-shell-execute

* src/w32fns.c (Fw32_shell_execute): Don't use filename_to_utf16
and filename_to_ansi to convert the DOCUMENT argument, as it could
be a URL that is not limited to MAX_PATH characters.  Instead, use
MultiByteToWideChar directly, and allocate heap storage as
required to accommodate the converted string.  Likewise with
non-Unicode operation.  Ensure OPERATION is null-terminated, even
if it is longer than 32K bytes.  (Bug#21158)