Fix menu placement on multiple-display setups when using lwlib

Fix menu placement on multiple-display setups when using lwlib

* lwlib/xlwmenu.c (fit_to_screen):
(pop_up_menu): Adjust menu position based on dimensions of the
current monitor's workarea.  (bug#52809)

* src/xfns.c (x_get_monitor_attributes): Stop testing for the
RandR extension here.
(xlw_monitor_dimensions_at_pos_1):
(xlw_monitor_dimensions_at_pos): New functions.

* src/xterm.c (x_term_init): Query for the RandR extension when
connecting to a display.
* src/xterm.h (xlw_monitor_dimensions_at_pos): New prototype.