coredump: capture crashing thread ID and name

coredump: capture crashing thread ID and name

Add %I (TID in initial PID namespace) to the core_pattern, so the
kernel passes the crashing thread's TID to systemd-coredump. Use it
to read the thread's comm name from /proc/<tid>/comm and log both as
new journal fields:

  COREDUMP_TID=       — TID of the crashing thread
  COREDUMP_THREAD_NAME= — comm name of the crashing thread

These fields are also stored as xattrs on external coredump files
(user.coredump.tid, user.coredump.thread_name) and displayed by
coredumpctl info alongside the PID line.

For single-threaded processes the TID equals the PID and thread_name
equals comm; for multi-threaded programs with named worker threads
(pthread_setname_np / PR_SET_NAME) this identifies which thread
crashed without needing to open the coredump file itself.

The new fields are optional in the socket forwarding path, so older
systemd-coredump senders are handled gracefully.

Co-developed-by: Claude <claude@anthropic.com>