Tidy up address and pointer handling and add debug for AVR.

	* avr-tdep.c: Improved commenting of address and code pointer
	usage.
	(AVR_CODE_START): Replaces AVR_IMEM_START.
	(AVR_DATA_START): Replaces AVR_SMEM_START.
	(AVR_EEPROM_START, AVR_EEPROM_MASK): Created.
	(avr_debug): Static variable at top level created.
	(avr_make_iaddr, avr_convert_iaddr_to_raw, avr_make_saddr)
	(avr_convert_saddr_to_raw): Deleted.
	(avr_strip_flag_bits, avr_make_code_addr, avr_make_code_ptr)
	(avr_make_data_addr, avr_make_data_ptr): Created.
	(avr_address_to_pointer, avr_pointer_to_address): Add optional
	debug output.
	(avr_integer_to_address): Use avr_make_data_addr instead of
	avr_make_saddr.
	(avr_read_pc, avr_unwind_pc): Explicitly OR in code bit instead of
	calling avr_make_iaddr.
	(avr_frame_unwind_cache, avr_unwind_sp, avr_dummy_id): Use
	avr_make_data_addr instead of avr_make_saddr.
	(avr_push_dummy_call): avr_strip_flag_bits instead of
	avr_convert_iaddr_to_raw.
	(_initialize_avr_tdep): add set/show debug avr as new command.