Support `select-active-regions' with xterm

Support `select-active-regions' with xterm

This allows Emacs to save the active region to the user's
primary selection on supported terminals.  The behavior follows
the existing `select-active-regions' variable and requires
`xterm-select-active-regions' to be non-nil.

* src/keyboard.c (command_loop_1):
* lisp/frame.el (display-selections-p): On text terminals, check
terminal parameter `xterm--set-selections' and variable
`xterm-select-active-regions' when deciding whether to update
primary selection. (bug#55883)
* lisp/term/xterm.el (xterm-select-active-regions): New
defcustom.