Add package files for libc, libedit, librtld_db, and libthr.

libedit is needed for sh(1), which if updated before runtime,
can cause undesirable behavior.

For the rest, the installation order needs to be:

1. librtld
2. libc
3. libthr

The dependency listing and shilbs_required entries ensure this
behavior.

Sponsored by: The FreeBSD Foundation