licenses & mk/license.mk: add license for Enlightenment 16

Enlightenment 16 uses a modified (non-standard) MIT license that
includes an advertising clause. (This makes it incompatible with the
GPL.) I've named it enlightenment16 to differentiate that Enlightenment
>=17 releases use the 2-Clause BSD. (Enlightenment 16 continues to be
developed independently, and is of current interest to pkgsrc users.)

In some places, this is referred to as the "MIT With Advertising"
license, but I'm not aware of other projects using this variant. If it
becomes more broadly relevant to pkgsrc, we could rename it such.

(This should have been added a long time ago, the wm/enlightenment
package simply has never had a LICENSE variable set. Better late than
never.)

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