List checkpoints in ascending order

List checkpoints in ascending order

Before:
     (gdb) info checkpoints
       3 process 29132 at 0x4008ad, file foo.c, line 81
       2 process 29131 at 0x4008ad, file foo.c, line 81
       1 process 29130 at 0x4008ad, file foo.c, line 81
     * 0 Thread 0x7ffff7fc5740 (LWP 29128) (main process) at 0x4008ad, file foo.c, line 81

After:
     (gdb) info checkpoints
     * 0 Thread 0x7ffff7fc5740 (LWP 29128) (main process) at 0x4008ad, file foo.c, line 81
       1 process 29130 at 0x4008ad, file foo.c, line 81
       2 process 29131 at 0x4008ad, file foo.c, line 81
       3 process 29132 at 0x4008ad, file foo.c, line 81

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.