stale-symlink-buildworld.sh: a script to check for stale symlinks on a FreeBSD system

You can run the script before or after `make installworld'

You may also check your local ports with:

env STALE_SYMLINK_BUILDWORLD_DIRS=/usr/local ./stale-symlink-buildworld.sh

PR: 276235

(cherry picked from commit e880dd644f63fbe068c38b73b44aa7e7c5f176f3)