Use a separate script to authenticode sign files.

* build-aux/authenticode-sign.sh: New.
* src/Makefile.am (stamp-additional-signedfiles): Call this tool.
* src/gpg4win.mk.in: Remove most of calls of READ_AUTOGEN_template.
(AUTHENTICODE_sign): Remove this macro.
(signed_installers): Call the new tool

The problem with the old use was that it was used in one place in an
eval and at another place directly. Thus the quoting required to be
different. A separate tools sounds better and can be extended for
other purposes. Note the tool's option --template.

Only the card based operation has been tested; the old signing on a
Windows host might be broken.