Make sure the 'tsget' script is called 'tsget.pl' everywhere

The result is that we don't have to produce different names on
different platforms, and we won't have confusion on Windows depending
on if the script was built with mingw or with MSVC.

Partial fix for #3254

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6764)