tekhex buffer management and symbol types

tekhex buffer management and symbol types

Dramatically reduces memory consumption and processing time for large
all-zero data segments.  Allows multiple symbol types attached to a
given segment to survive objcopy.

	* tekhex.c (CHUNK_SPAN): Define.
	(struct data_struct <chunk_init>): Use one byte per span, update
	all code accessing this field.
	(find_chunk): Add create param, don't create new entry unless set.
	(insert_byte): Don't save zeros.
	(first_phase): Set section SEC_CODE or SEC_DATA flag depending
	on symbol type.  Create an alternate section if both types of
	symbol are given.  Attach type '2' and '6' symbols to absolute
	section.
	(move_section_contents): Fix caching of chunk.  Don't create chunk
	when reading, or for writing zeros.
	(tekhex_set_section_contents): Don't create initial chunks.
	(tekhex_write_object_contents): Use CHUNK_SPAN.