Various cleanups in target read/write code

Various cleanups in target read/write code

This contains various cleanups in the target memory read and write code.
They are not directly related to the non-8-bits changes, but they
clarify things a bit down the line.

gdb/ChangeLog:

	* target.c (target_read): Rename variables and use
	TARGET_XFER_E_IO.
	(target_read_with_progress): Same.
	(read_memory_robust): Constify parameters and rename
	variables.
	(read_whatever_is_readable): Constify parameters,
	rename variables, adjust formatting.
	* target.h (read_memory_robust): Constify parameters.