Ardour prevents repeat crashes by first marking a plugin
as blacklisted, and after a successful scan the plugin is
whitelisted.
When not scanning for plugins, but reading the cache
and successfully parsing the cashed content, a VST2 plugin
is whitelisted, even though it may not have been blacklisted.
In case the blacklist is empty, this lead to a logging:
"Expected VST Blacklist file does not exist."
PS. This also consolidates VST2 + VST3 code for white/blacklist
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