registry: add support for GST_REGISTRY_REUSE_PLUGIN_SCANNER=no

registry: add support for GST_REGISTRY_REUSE_PLUGIN_SCANNER=no

This will make sure we spawn a new plugin scanner helper for each plugin
to be introspected, which helps with making sure we don't load too many
shared objects (libs, plugins) at the same time on systems where there
is a hard limit like on Android.

A better version might re-use the scanner for up to N times, though
it's not clear whether that would actually improve things dramatically.

https://bugzilla.gnome.org/show_bug.cgi?id=662091