Adapt to new USE_GITHUB features.

Add LICENSE_FILE.

Add sie-nmsg to RUN_DEPENDS.

Don't bother with install-strip because it was not stripping libaxa.so,
and sratool and sratunnel are stripped even without it. Use ${STRIP_CMD}
for the library.

Differential Revision: https://reviews.freebsd.org/D2233
Approved by: mat (mentor)
Sponsored by: Farsight Security, Inc.