Implement alternative sub-buffer support in term.el

Implement alternative sub-buffer support in term.el

* etc/e/eterm-color.ti: Added termcaps for entering and leaving an
alternative sub-buffer.
* lisp/term.el (term-reset-size): Resize correctly in an alternative
sub-buffer is in use.
(term-clear-full-screen-programs): New user option.
(term-handle-ansi-escape):
(term-termcap-format): Handle termcaps for entering and leaving an
alternative sub-buffer.
(term-switch-to-alternate-sub-buffer): New (used to be commented out)
function to enter or leave an alternative sub-buffer.
bug#53485