Add 4BSD
Specify how to merge 4BSD with SCCS
Merge branch 'master' of github.com:dspinellis/unix-history-make

Conflicts:
	src/import.sh
Improve SCCS merge with 4BSD

- Prepend /usr/src to SCCS file path
- Include reference copy of 3BSD in SCCS import
- Verify 4BSD import
- Verify blame after SCCS merge
- Update README