If no instance directory specified, then use paths like
/etc/serval/serval.conf
/var/cache/serval
/var/log/serval
/var/run/serval
etc. for files, instead of all in a single directory.
Log all directory creation as INFO messages.
Interpretation of log.file.directory_path has changed slightly.
Updated servald configuration tech doc.
Replace form_serval_instance_path(), which takes a single 'path'
argument, with formf_serval_instance_path() and
vformf_serval_instance_path(), which take printf(3)-like arguments.
This supports more flexible instance path file names.
FORM_SERVAL_INSTANCE_PATH() macro works as before, but reimplemented
