Improve handling of pool_options::largest_required_pool_block

Improve handling of pool_options::largest_required_pool_block

Make the munge_options function round the largest_required_pool_block
value to a multiple of the smallest pool size (currently 8 bytes) to
avoid pools with odd sizes.

Ensure there is a pool large enough for blocks of the requested size.
Previously when largest_required_pool_block was exactly equal to one of
the pool_sizes[] values there would be no pool of that size. This patch
increases _M_npools by one, so there is a pool at least as large as the
requested value. It also reduces the size of the largest pool to be no
larger than needed.

	* src/c++17/memory_resource.cc (munge_options): Round up value of
	largest_required_pool_block to multiple of smallest pool size. Round
	excessively large values down to largest pool size.
	(select_num_pools): Increase number of pools by one unless it exactly
	matches requested largest_required_pool_block.
	(__pool_resource::_M_alloc_pools()): Make largest pool size equal
	largest_required_pool_block.
	* testsuite/20_util/unsynchronized_pool_resource/options.cc: Check
	that pool_options::largest_required_pool_block is set appropriately.

From-SVN: r266089