config-parser: Store service directories in structs

config-parser: Store service directories in structs

This lets us give them a flags word, which we immediately use to
track whether this directory should be watched with inotify or
equivalent.

The struct name is unfortunately a bit odd, because I had aimed to
use BusServiceDir, but activation.c already has BusServiceDirectory
so that would have been too confusing.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99825
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>