Constify some commands in record.c

Constify some commands in record.c

gdb/ChangeLog
2017-09-27  Tom Tromey  <tom@tromey.com>

	* record.c (cmd_record_delete, cmd_record_stop, cmd_record_save)
	(cmd_record_goto_begin, cmd_record_goto_end, get_insn_number)
	(get_context_size, no_chunk, get_insn_history_modifiers)
	(cmd_record_insn_history, get_call_history_modifiers)
	(cmd_record_call_history): Constify.