net-mgmt/ipcad: touchup and build fixes

 - fix wrong test(1) option in bpf configure test
   preventing configuration to succeed when no bpf(4)
   device is present on the system
 - add a missing include to avoid undefined behaviour
   through homegrown artisanal offsetof(3) macro
 - set LICENSE

PR:		265512
Approved by:	vlm@lionet.info (maintainer timeout)