Improve handling of plugin functions of the same name

The shortname of a plugin function can be the same for several
functions.  This commit introduces some priority rules for chosing
the function the shortname is assigned to:

1. no excluded from autocomplete > excluded
2. don't override user script functions
3. autoloaded > filterdb > added by the user
4.1 AviSynth lookup order (autoloaded)
4.2 alphabetical (filterdb)
4.3 undefined (added by the user)