List displays in ascending order

List displays in ascending order

Before:
      (gdb) info display
      Auto-display expressions now in effect:
      Num Enb Expression
      3:   y  1
      2:   y  1
      1:   y  1

After:
      (gdb) info display
      Auto-display expressions now in effect:
      Num Enb Expression
      1:   y  1
      2:   y  1
      3:   y  1

gdb/ChangeLog:
2015-11-24  Pedro Alves  <palves@redhat.com>

	PR 17539
	* printcmd.c (display_command): Append new display at the end of
	the list.

gdb/testsuite/ChangeLog:
2015-11-24  Pedro Alves  <palves@redhat.com>

	PR 17539
	* gdb.base/display.exp: Expect displays to be sorted in ascending
	order.  Use multi_line.
	* gdb.base/solib-display.exp: Likewise.