cgroup-util: Don't try to open pidfd for pids from cgroup.threads

Opening pidfds for non thread group leaders only works from 6.9 onwards with PIDFD_THREAD. On
older kernels or without PIDFD_THREAD pidfd_open() fails with EINVAL. Since we might read non
thread group leader IDs from cgroup.threads, we introduce and set CGROUP_NO_PIDFD to avoid
trying open pidfd's for them and instead use the pid as is.

src/basic/cgroup-util.h

@@ -188,6 +188,7 @@ typedef enum CGroupFlags {
         CGROUP_IGNORE_SELF        = 1 << 1,
         CGROUP_REMOVE             = 1 << 2,
         CGROUP_DONT_SKIP_UNMAPPED = 1 << 3,
+        CGROUP_NO_PIDFD           = 1 << 4,
 } CGroupFlags;
 
 int cg_enumerate_processes(const char *controller, const char *path, FILE **ret);