Tidy up term.el menu handling

Tidy up term.el menu handling

* lisp/term.el: (term-mode-map): Use easymenu for In/Out, Complete menus.
(term-pager-break-map): Initialize in the defvar.
(term-terminal-menu, term-signals-menu): Define with easymenu.
(term-terminal-menu): Also show it in line-mode.
(term-pager-menu): New, extracted from term-process-pager.
(term-mode, term-char-mode, term-process-pager): Use easymenu-add.
(term-update-mode-line): Propertize line/char and page items.
(term-process-pager): Move keymap initialization elsewhere.

Fixes: debbugs:11957