Implement -exec-continue/-exec-interrupt --all.

	Implement -exec-continue/-exec-interrupt --all.
        * infcmd.c (continue_1): New, extracted from
        (continue_command): ...here.
        (interrupt_target_1): New, extracted from
        (interrupt_target_command): ...here.
        * inferior.h (continue_1, interrupt_target_1): New.
        * mi/mi-main.c (mi_cmd_exec_continue)
        (mi_cmd_exec_interrupt): Handle --all.