net/traff: fix build with -fno-common

Also:

• define LICENSE
• fix missing LIB_DEPENDS on libargp.so

PR: 267594
Reported by: fuz@fuz.su
Approved by: daniel@freebsd.ro (maintainer, timeout 2 weeks)
MFH: 2022Q4 (build fix)

(cherry picked from commit 98a1bc7f0d2f5539100fe22e27f83cedb29ab040)