tree-wide: Add error prefixing for most remaining syscalls

There were some important ones there like a random `syncfs()`. The remaining
users are mostly blocked on the "fstatat enoent" case, I'll wait to port those.

Closes: #1150
Approved by: jlebon

src/libostree/ostree-sepolicy.c

@@ -521,7 +521,7 @@ ostree_sepolicy_get_label (OstreeSePolicy    *self,
       if (errno == ENOENT)
         *out_label = NULL;
       else
-        return glnx_throw_errno (error);
+        return glnx_throw_errno_prefix (error, "selabel_lookup");
     }
   else
     {