Fix incorrect reporting of memory on OS X via memsup.

Application memsup should be calculating free memory using the
speculative pages, in the same manner that the Activity Monitor and top
programs on OS X do.  In addition, correct page size to 4096, based on
verification of available memory between top, vm_stat and Activity
Monitor.

References:

http://superuser.com/questions/197059/mac-os-x-sysctl-get-total-and-free-memory-size
http://erlang.org/pipermail/erlang-questions/2009-March/042201.html
http://apple.stackexchange.com/questions/81581/why-does-free-active-inactive-speculative-wired-not-equal-total-ram
http://superuser.com/questions/197059/mac-os-x-sysctl-get-total-and-free-memory-size#comment200035_197086