Do not register previously registered plugins

It may happen that a plugin listed as enabled in the plugin table is in
fact already registered with MantisBT if that plugin is also listed as
force_installed in the configuration.

In that case, re-registering it basically 'unprotects' it, and
manage_plugin_page.php displays an Uninstall action even though the
plugin can't actually be uninstalled.
Fix data type issues in plugin API

- enabled and protected are boolean
- priority is int

The type cast before assignment ensures we don't get type mismatch
errors in check_selected/check_checked functions, since DB api seems to
return these fields as strings.
New API to return list of force-installed plugins
Remove unnecessary MantisCore-specific code

Using the new plugin_get_force_installed() function (which always
returns 'MantisCore') allows simplification of several plugin API, as
they no longer require specific code to handle the pseudo plugin.
Simplify code to list installed plugins
Coding guidelines