lib/sysroot: Add journal-msg signal

This will allow us to drop the awful hack in rpm-ostree where we watch our own
stdout. In general, libraries shouldn't write to stdout.

Also we can kill the systemd journal wrapper code. There's some duplication at
each call site now...but it's easier than trying to write a `sd_journal_send()`
wrapper.

I was originally going to have this emit all of the structured data too as a
`GVariant` but decided it wasn't worth it right now.

Closes: #1052
Approved by: jlebon

src/libostree/ostree-sysroot-private.h

@@ -69,6 +69,10 @@ struct OstreeSysroot {
 #define _OSTREE_SYSROOT_DEPLOYMENT_RUNSTATE_DIR "/run/ostree/deployment-state/"
 #define _OSTREE_SYSROOT_DEPLOYMENT_RUNSTATE_FLAG_DEVELOPMENT "unlocked-development"
 
+void
+_ostree_sysroot_emit_journal_msg (OstreeSysroot  *self,
+                                  const char     *msg);
+
 gboolean
 _ostree_sysroot_read_boot_loader_configs (OstreeSysroot *self,
                                           int            bootversion,