ulogd: add ulogd_parse_configfile public function

ulogd: add ulogd_parse_configfile public function

Provide a new function `ulogd_parse_configfile()` in the public
interface, which wraps `parse_config_file()` to parse a section of the
config file and communicates found errors to the user. It can be used
as a drop-in replacement because arguments and return value are
compatible.

This relieves plugins of the need to translate the individual error
codes to human readable messages, and plugins are mostly interested if
there is any error, not what specific error.

This reuses the existing `parse_conffile()` function with slight
adjustments.

Signed-off-by: Corubba Smith <corubba@gmx.de>
Signed-off-by: Florian Westphal <fw@strlen.de>