lib/deploy: Support locking deployment finalization

Teach `ostree-finalize-staged.service` to check for a file in `/run` to
determine if it should do the finalization. This will be used in
RPM-OSTree, where we want to be able to separate out "preparing updates"
from "making update the default" for more fine-grained control. See:
https://github.com/projectatomic/rpm-ostree/issues/1748

Closes: #1841
Approved by: cgwalters

src/libostree/ostree-sysroot-private.h

@@ -76,6 +76,7 @@ struct OstreeSysroot {
 #define OSTREE_SYSROOT_LOCKFILE "ostree/lock"
 /* We keep some transient state in /run */
 #define _OSTREE_SYSROOT_RUNSTATE_STAGED "/run/ostree/staged-deployment"
+#define _OSTREE_SYSROOT_RUNSTATE_STAGED_LOCKED "/run/ostree/staged-deployment-locked"
 #define _OSTREE_SYSROOT_DEPLOYMENT_RUNSTATE_DIR "/run/ostree/deployment-state/"
 #define _OSTREE_SYSROOT_DEPLOYMENT_RUNSTATE_FLAG_DEVELOPMENT "unlocked-development"