breakpoint.c:commands_command_1 constification and cleanup

breakpoint.c:commands_command_1 constification and cleanup

This is constification needed for next patch.  Adjust
commands_command_1 to use std::string too because the "arg" parameter
is currently overwritten and then passed to make_cleanup.  The
constification alone would trigger a compile error in the make_cleanup
call otherwise (passing const char * to void * parameter).  Using
std::string gets rid of the cleanup in the first place, resulting in
simpler code.

gdb/ChangeLog:
2016-11-08  Pedro Alves  <palves@redhat.com>

	* breakpoint.c (struct commands_info) <arg>: Constify.
	(commands_command_1): Constify 'arg' parameter.  Use std::string
	and string_printf.
	(commands_from_control_command): Constify 'arg' parameter.
	(map_breakpoint_numbers): Constify 'args' parameter.
	* breakpoint.h (commands_from_control_command): Constify 'arg'
	parameter.