This introduces the predicate frame-initial-p, which uses
struct frame.output_method or struct terminal.type to detect
initial_terminal without relying on its name (bug#80629).
For some prior discussion, see:
https://lists.gnu.org/r/emacs-devel/2019-12/msg00480.html
https://lists.gnu.org/r/emacs-devel/2020-01/msg00120.html

* doc/lispref/frames.texi (Frames): Document frame-initial-p.
(Finding All Frames): Fix grammar.
* etc/NEWS (Lisp Changes in Emacs 31.1): Announce frame-initial-p.
* lisp/desktop.el (desktop--check-dont-save):
* lisp/emacs-lisp/debug.el (debug):
* lisp/frameset.el (frameset-restore):
* lisp/menu-bar.el (menu-bar-update-buffers):
* lisp/xt-mouse.el (turn-on-xterm-mouse-tracking-on-terminal):
Use frame-initial-p instead of checking the "initial_terminal" name.
* lisp/emacs-lisp/byte-opt.el: Mark frame-initial-p as error-free.

* src/pgtkterm.c (pgtk_focus_changed): Use IS_DAEMON in place of
Fdaemonp, thus also accepting a named daemon session.
* src/terminal.c (decode_tty_terminal): Clarify commentary.
(Fframe_initial_p): New function.
(syms_of_terminal): Expose it.
(init_initial_terminal): Update commentary now that
menu-bar-update-buffers uses frame-initial-p (bug#53740).

* test/lisp/xt-mouse-tests.el (with-xterm-mouse-mode): Simulate the
lack of an initial terminal by overriding frame-initial-p now
that turn-on-xterm-mouse-tracking-on-terminal uses it.
* test/src/terminal-tests.el: New file.