Clean up Motif drag-and-drop code

Clean up Motif drag-and-drop code

* src/xterm.c (enum xm_drag_operation, enum xm_drag_action)
(enum xm_drag_reason, enum xm_drag_originator, enum xm_drag_style)
(enum xm_drop_site_status): Turn macros into enums.
(x_next_event_from_any_display): Fix initial value of rc.