stringmerge: don't presize hash table

stringmerge: don't presize hash table

originally the reason for pre-sizing was that that's easier
for a multi-threaded use of the hash table.  That hasn't materialized
yet, so there's not much sense in using the very very conservative
estimates for pre-sizing.  Doing the resize on-demand, whenever we
actually need to add a new entry doesn't change performance.

	bfd/
	merge.c (sec_merge_hash_insert): Resize as needed from here ...
	(record_section): ... not from here.  Don't calculate estimates,
	return bool instead of three-state, regard all errors as soft
	errors.
	(_bfd_merge_sections): Adjust.