gdb: print thread names in thread apply command output

This makes the thread apply command print the thread's name.  The use
of target_pid_to_str is replaced by thread_target_id_str, which
provides the same output as "info threads".

Before:
(gdb) thread apply 2 bt

Thread 2 (Thread 0x7fd245602700 (LWP 3837)):
[...]

After:
(gdb) thread apply 2 bt

Thread 2 (Thread 0x7fd245602700 (LWP 3837) "HT cleanup"):
[...]

The thread's description header is pre-computed before running the
command since the command may change the selected inferior. This is
not permitted by thread_target_id_str as target_thread_name asserts
that `info->inf == current_inferior ()`.

This situation arises in the `gdb.threads/threadapply.exp` test which
kills and removes the inferior as part of a "thread apply" command.

gdb/ChangeLog:

        * thread.c (thr_try_catch_cmd): Print thread name.