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

src/libotutil/otutil.h

@@ -47,5 +47,6 @@
 #include <ot-spawn-utils.h>
 #include <ot-checksum-utils.h>
 #include <ot-gpg-utils.h>
+#include <ot-log-utils.h>
 
 void ot_ptrarray_add_many (GPtrArray  *a, ...) G_GNUC_NULL_TERMINATED;