Add scripts to enable / disable keyboxd

* src/Makefile.am (EXTRA_DIST): Update accordingly.
* src/gpg-disable-keyboxd.bat, src/gpg-enable-keyboxd.bat: New.
* src/inst-gnupg-w32.nsi: Install to gnupg/bin to have in PATH.
* src/uninst-gnupg-w32.nsi: Uninstall them.

This should help get more feedback if users can easily try
out the benefits of keyboxd. Worst case the migration back
does not work and they will lose imported keys between the
activation of keyboxd and the state of pubring.kbx.

• GnuPG-Bug-Id: T6589