Better error message for non-existent generics groups.

Closes PR13671.
redex: bug fix for gerating terms including #f

please include in the release
fix a non-deterministic test
JIT for ARM

Includes switch from LGPL 2.1 to LGPL 3, since the ARM port
of GNU lightining is licensed under LGPL 3.
adjust pick-a-maze based on Neil's comments

please include in the release
add missing license file
Tests for UDP multicast primitives