(message_dolog_marker1, message_dolog_marker2)

(message_dolog_marker1, message_dolog_marker2)
(message_dolog_marker3): New static variables hold three markers.
(syms_of_xdisp): Initialize and staticpro them.
(message_dolog): Use message_dolog_marker1..3 instead of
allocating markers each time.  Unchain them when done.