core: add support for pidfd_spawn

Added in glibc 2.39, allows cloning into a cgroup and to get
a pid fd back instead of a pid. Removes race conditions for
both changing cgroups and getting a reliable reference for the
child process.

Fixes https://github.com/systemd/systemd/pull/18843
Replaces https://github.com/systemd/systemd/pull/16706

src/basic/process-util.h

@@ -238,7 +238,7 @@ int get_process_threads(pid_t pid);
 int is_reaper_process(void);
 int make_reaper_process(bool b);
 
-int posix_spawn_wrapper(const char *path, char *const *argv, char *const *envp, PidRef *ret_pidref);
+int posix_spawn_wrapper(const char *path, char *const *argv, char *const *envp, const char *cgroup, PidRef *ret_pidref);
 
 int proc_dir_open(DIR **ret);
 int proc_dir_read(DIR *d, pid_t *ret);