Refactor 'maint time' command statistics.

Refactor 'maint time' command statistics.

Consolidate code for displaying per-command time and space statistics to avoid
duplication.  Piggyback on cleanups so that statistics get printed even when
commands terminate as a result of an error.

Changelog

    * gdb/defs.h (make_command_stats_cleanup): Declare.
    (set_display_time): Declare.
    (set_display_space): Declare.
    * gdb/event-top.c (command_handler): Use make_command_stats_cleanup.
    * gdb/main.c (display_time, display_space): Move definitions to utils.c.
    (captured_main): Use make_command_stats_cleanup to get start-up
    statistics.
    Use set_display_time and set_display_space for processing OPT_STATISTICS
    case.
    * gdb/maint.c (maintenance_time_display): Use set_display_time.
    (maintenance_space_display): Use set_display_space.
    * gdb/top.c (execute_command): Remove obsolete 'maint time' code.
    (command_loop): Use make_command_stats_cleanup.
    * gdb/utils.c (struct cmd_stats): Structure for storing initial time
    and space usage.
    (display_time, display_space): Move definitions here from utils.c.
    (set_display_time): New function.
    (set_display_space): New function.
    (make_command_stats_cleanup): New function.
    (report_command_stats): New auxiliary function for
    make_command_stats_cleanup.
    * gdb/testsuite/gdb.gdb/selftest.exp: Adjust expected message for
    capturing start-up runtime.