Support files for launching dinit from /etc/rc.d/ on FreeBSD

To install dinit on FreeBSD, run the following commands as root:

```
# (Optional) Install build dependencies
pkg install -y meson rsync

# Build dinit with correct path
pushd ../..
meson setup -Ddinit-control-socket-path=/var/run/dinitctl build2
cd build2
ninja install
popd

# Install dinit as rc.d service
rsync -r tree/ /
mkdir -p /etc/dinit.d/boot.d

# enable dinit (to start on next boot)
/etc/rc.d/dinit enable
# start dinit now
/etc/rc.d/dinit start
```