This patch makes gdb, when target memory map is available, optionally reject 
all memory accesses outside of regions in memory map. The rationale is that:

	1. It's sometimes better to get error that have the write silently do
        nothing or read returning garbase.

	2. For some targets, accesses to non-existent memory "wedge" them.

Since memory maps are still rare, this behaviour is enabled only by a new 
command.

OK?

- Volodya

	* memattr.h (enum mem_access_mode): New value
	MEM_NONE.
	* memattr.c (unknown_mem_attrib): New.
	(inaccessible_by_default): New.
	(show_inaccessible_by_default): New.
	(lookup_mem_region): Check inaccessible_by_default.
	(dummy_cmd): New.
	(mem_set_cmdlist, mem_show_cmdlist): New.
	(_initialize_mem): Register new "set" and "show"
	commands.
	* target.c (memory_xfer_partial): If memory type
	is MEM_NONE, return an error.
	Clip to region size when calling to_xfer_partial.
	If upper limit of memory range is 0, don't clip
	anything.