CLEANUP: ebtree: clarify licence and update to 6.0.6

CLEANUP: ebtree: clarify licence and update to 6.0.6

(from ebtree 6.0.6)

This version is mainly aimed at clarifying the fact that the ebtree license
is LGPL. Some files used to indicate LGPL and other ones GPL, while the goal
clearly is to have it LGPL. A LICENSE file has also been added.

No code is affected, but it's better to have the local tree in sync anyway.

(cherry picked from commit 24dc7cca051f081600fe8232f33e55ed30e88425)