lib: Use sd_journal directly (optionally)

lib: Use sd_journal directly (optionally)

This was the last caller of libgsystem that isn't
`gs_file_get_path_cached()`.  I think the use case ostree has where
the same code can be called via command line and via a shared library
*and* via a daemon is rather unusual, so let's just copy the code for
logging from libgsystem into here.

For example rpm-ostree hard depends on a daemon mode, so it'll just
use `sd_journal` directly.

Closes: #341
Approved by: jlebon