Change the "opt.lg_prof_sample" default from 0 to 19 (1 B to 512 KiB).

Change the "opt.prof_accum" default from true to false.

Add the "opt.prof_final" mallctl, so that "opt.prof_prefix" need not be
abused to disable final profile dumping.

Start preparing ChangeLog for 3.0.0 release.  Additional fixes and
changes are yet to come, so this is not a complete ChangeLog.

Fix indentation inconsistencies in ChangeLog.

Add ChangeLog, which briefly summarizes releases.

Edit README and INSTALL.