Capitalize output of successful checkpoint command

Capitalize output of successful checkpoint command

This commit causes the output of a "checkpoint" command to be
changed from:

    checkpoint N: fork returned pid DDDD

to:

    Checkpoint N: fork returned pid DDDD

This change was made to bring the output of the "checkpoint" command in
line with that of other commands, e.g.:

    (gdb) break main
    Breakpoint 1 at ...

    (gdb) catch exec
    Catchpoint 2 (exec)

    (gdb) add-inferior
    [New inferior 2]
    Added inferior 2

The tests gdb.base/checkpoint.exp, gdb.base/kill-during-detach.exp,
and gdb.multi/checkpoint-multi.exp have been updated to accept the new
(capitalized) output from the "checkpoint" command.

Reviewed-By: Tom Tromey <tom@tromey.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>