(message_truncate_lines, Qmessage_truncate_lines): New

(message_truncate_lines, Qmessage_truncate_lines): New
variables.
(ensure_echo_area_buffers): Initialize echo buffer's
truncate lines setting to nil.
(with_echo_area_buffer): Don't set the echo buffer's truncate
lines setting here.
(set_message_1): Set it here, instead, based on the value
of message_truncate_lines.
(resize_mini_window): Handle case that lines are truncated.
(syms_of_xdisp): Initialize Qmessage_truncate_lines.  DEFVAR_BOOL
message-truncate-lines.