Show backtraces of threads from thread list buffer

Show backtraces of threads from thread list buffer

* src/eval.c (backtrace_thread_p, backtrace_thread_top)
(backtrace_thread_next, Fbacktrace_frames_from_thread): New functions.
* lisp/thread.el (thread-list-mode-map): Add keybinding and
menu item for 'thread-list-pop-to-backtrace'.
(thread-list-mode): Make "Thread Name" column wide enough
for the result of printing a thread with no name with 'prin1'.
(thread-list--get-entries): Use 'thread-list--name'.
(thread-list--send-signal): Remove unnecessary calls to 'threadp'.
(thread-list-backtrace--thread): New variable.
(thread-list-pop-to-backtrace): New command.
(thread-list-backtrace--revert-hook-function)
(thread-list--make-backtrace-frame)
(thread-list-backtrace--insert-header, thread-list--name): New
functions.