core: mount namespaces: Remove auxiliary bind mounts directory after unit termination

Unit that requires its own mount namespace creates a temporary directory
to implement dynamic bind mounts (org.freedesktop.systemd1.Manager.BindMountUnit).
However, this directory is never removed and they will accumulate for
each unique unit (e.g. templated units of systemd-coredump@).

Attach the auxiliary runtime directory existence to lifetime of other
"runtime" only per-unit directories.

src/core/execute.h

@@ -459,6 +459,7 @@ void exec_context_dump(const ExecContext *c, FILE* f, const char *prefix);
 
 int exec_context_destroy_runtime_directory(const ExecContext *c, const char *runtime_root);
 int exec_context_destroy_credentials(const ExecContext *c, const char *runtime_root, const char *unit);
+int exec_context_destroy_mount_ns_dir(Unit *u);
 
 const char* exec_context_fdname(const ExecContext *c, int fd_index);