COPYING: Now fully LGPLv2+

I ran into Jeremy Katz today, and he gave me permission to relicense
the small bits of switch-root.c to LGPLv2+.  This combined with
permission from Peter Jones allows OSTree to become fully LGPLv2+.

Not a big deal, it's just a lot clearer to only have one license, and
it makes it easier to turn application code into library code.