Add add-plugin and remove-plugin; maintain an Info index.

Add add-plugin and remove-plugin; maintain an Info index.

The postrm Debian installation scripts do not work if they are run
after the core package is removed.  And prerm scripts do not work if
they update indexes based on what is installed.  (The package being
removed is still installed.)  Replace update-html-index and update-
optiondb-index with add-plugin and remove-plugin, procedures that
add/remove names to/from a list.  These work in prerm scripts.