[9.18] new: usr: Print the full path of the working directory in startup log messages

named now prints its initial working directory during startup and the
changed working directory when loading or reloading its configuration
file if it has a valid 'directory' option defined.

Closes #4731

Backport of MR !9362

Merge branch 'backport-4731-log-workdir-full-path-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9373