Add the "stats.cactive" mallctl, which can be used to efficiently and
repeatedly query approximately how much active memory the application is
utilizing.
Rather than blindly assigning threads to arenas in round-robin fashion,
choose the lowest-numbered arena that currently has the smallest number
of threads assigned to it.
Add the "stats.arenas.<i>.nthreads" mallctl.
Convert the man page source from roff to DocBook, and generate html and
roff output. Modify the build system such that the documentation can be
built as part of the release process, so that users need not have
DocBook tools installed.
