net/wackamole: Fix build on armv7

Jails may not have a bpf(4) device, but can still build with BPF.

• fix the configure test for BPF detection
• define LICENSE
• fix two obvious bugs in the code

PR: 265561
Approved by: maintainer timeout, >2 weeks
MFH: 2022Q3

(cherry picked from commit 4d98b01b6fff7b4c1dd36d5f0849ed6dc1f3eb9e)