library: address memory aberration with LXC containers

Under a lxc container, the /proc/meminfo 'MemFree' and
'MemAvailable' amounts will be equal, unless memory is
being limited via cgroups in which case 'MemAvailable'
could exceed that for 'MemTotal'. And when a container
has been nested, there exist additional memory quirks.

A program might then display used or available amounts
greater than total memory (assuming unsigned honored),
or negative values (should a signed cast be employed).

This anomaly primarily impacted the top and free pgms.
Thus, two simple sanity checks have been introduced to
avoid any illogical kb_main_available or kb_main_used.

( Busybox top & free also display anomalous although )
( different results when running in a lxc container. )

Reference(s):
https://bugzilla.redhat.com/show_bug.cgi?id=1153817



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