[RT#91447] Add PPM_UNINSTALL_EXEC and PPM_UNINSTALL_SCRIPT

It would be nice to complement support for PPM_INSTALL_EXEC / PPM_INSTALL_SCRIPT with their uninstall
counterparts, let's say PPM_UNINSTALL_EXEC + PPM_UNINSTALL_SCRIPT.

According to the PPD specs, the XML tag <UNINSTALL> can be specified to run a pre-deinstall script.

Signed-off-by: Chris 'BinGOs' Williams <chris@bingosnet.co.uk>
Release engineering for v6.85_02