plugin: load plugins with unknown license strings

plugin: load plugins with unknown license strings

We shouldn't fail to load plugins just because we don't
recognise the license string. It's not our job to validate
licenses, and the license list is outdated and ambiguous
anyway.

Also add MPL-2.0 to the list, and fix some defunct license
URLs in the code comments.

Get rid of the hard-coded skip index, doesn't really buy us
much versus just skipping with a few strlens, and is harder
to maintain.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/864>