Modify allocation policy, in order to avoid excessive fragmentation for

Modify allocation policy, in order to avoid excessive fragmentation for
allocation patterns that involve a relatively even mixture of many
different size classes.

Reduce the chunk size from 16 MB to 2 MB.  Since chunks are now carved up
using an address-ordered first best fit policy, VM map fragmentation is
much less likely, which makes smaller chunks not as much of a risk.  This
reduces the virtual memory size of most applications.

Remove redzones, since program buffer overruns are no longer as likely to
corrupt malloc data structures.

Remove the C MALLOC_OPTIONS flag, and add H and S.