top: exploit new kb_main_available, make Jaromir happy

top: exploit new kb_main_available, make Jaromir happy

This patch will trade a former pessimistic calculation
of free physical memory for a more optimistic one that
uses the newly added kb_main_available library export.

But in case one might wish to return to the old former
method, there's a new #define that was made available.

[ the new calculation will affect graphing mode only ]

Reference(s):
http://www.freelists.org/post/procps/systemd-support-to-library,9

Signed-off-by: Jim Warner <james.warner@comcast.net>