Implement scrolling of TTY menus when the screen is too short.

Implement scrolling of TTY menus when the screen is too short.

 src/term.c (tty_menu_display): Accept an additional argument, the
 menu item from which to start displaying the menu.  Account for
 the value of Y when limiting the menu to the number of available
 screen lines.
 (mi_result): New enumeration.
 (read_menu_input): Return enumerated value.  When the y coordinate
 hits min_y or max_y, return scroll indication instead of wrapping
 around the menu.
 (tty_menu_activate): Handle the scrolling indications from
 read_menu_input.  Compute the first menu item to display and pass
 it to tty_menu_display.

 lisp/menu-bar.el (tty-menu-navigation-map): Bind shifted mouse clicks
 to commands that scroll the menu.