155: license: add GCC-exception-2.0 r=jmesmon a=jmesmon

All code included in rust-systemd that was not relicensed by the
contributors has either been removed, or corresponds to
non-copyrightable elements (direct translation of C api definitions into
rust).

The new code was written without reference to the removed code

Closes #92 and closes #127

Thanks everyone for your help on this!

Co-authored-by: Cody P Schafer <dev@codyps.com>

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