opt: relicense to GPLv2+ (from GPLv3+)

Acked by contributors:
Luke-Jr <luke@dashjr.org>:
	 I don't have any objections
Douglas Bagnall <douglas@halo.gen.nz>:
	Fine by me.
Joel Stanley <joel@jms.id.au>:
	Fine with me.
Brad Hards <bradh@frogmouth.net>:
	Ack.

No response from Joey (trivial warning patch) or Andreas Schlick (one
trivial patch, but he also wrote opt_free() (which was a 2 line function).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>