process-util: add helper get_process_threads()

Let's add a proper helper for querying the number of threads in a
process.

src/basic/process-util.h

@@ -189,3 +189,5 @@ int pidfd_verify_pid(int pidfd, pid_t pid);
 int setpriority_closest(int priority);
 
 _noreturn_ void freeze(void);
+
+int get_process_threads(pid_t pid);