library: tweak newly added MemAvailable fall-back code

library: tweak newly added MemAvailable fall-back code

This commit just ensures recalculation of some amounts
for iterative processes, like top. It also trades some
repeated runtime calls to sysconf for a one time cost.

Reference(s):
http://www.freelists.org/post/procps/systemd-support-to-library,7
. fall-back calculations
commit b779855cf15d68f9038ff1809db18c0788e9ae70

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