Easier and more stubborn MI memory read commands.

	Easier and more stubborn MI memory read commands.

	* mi/mi-cmds.c (mi_cmds): Register data-read-memory-bytes
	and data-write-memory-bytes.
	* mi/mi-cmds.h (mi_cmd_data_read_memory_bytes)
	(mi_cmd_data_write_memory_bytes): New.
	* mi/mi-main.c (mi_cmd_data_read_memory): Use regular target_read.
	(mi_cmd_data_read_memory_bytes, mi_cmd_data_write_memory_bytes):
	New.
	(mi_cmd_list_features): Add "data-read-memory-bytes" feature.
	* target.c (target_read_until_error): Remove.
	(read_whatever_is_readable, free_memory_read_result_vector)
	(read_memory_robust): New.
	* target.h (target_read_until_error): Remove.
	(struct memory_read_result, free_memory_read_result_vector)
	(read_memory_robust): New.