Constify commands in cli-dump.c

Constify commands in cli-dump.c

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

	* cli/cli-dump.c (dump_memory_command, dump_value_command)
	(dump_srec_memory, dump_srec_value, dump_ihex_memory)
	(dump_ihex_value, dump_verilog_memory, dump_verilog_value)
	(dump_tekhex_memory, dump_tekhex_value, dump_binary_memory)
	(dump_binary_value, append_binary_memory, append_binary_value):
	Constify.
	(struct dump_context) <func>: Constify.
	(add_dump_command): Update.