library: lift item restrictions on 'get()', <STAT> api

library: lift item restrictions on 'get()', <STAT> api

In the commit referenced below it was acknowledged the
procps_stat_get() function will intentionally restrict
returned values by excluding DELTA items among others.

Upon reflection that decision seemed rather arbitrary,
and so has now been lifted. And, while the initial use
of a DELTA will return 0, subsequent calls to that get
function will now return the difference (providing one
second, at least, has elapsed between any such calls).

Reference(s):
. reference to design decisions
commit abc71a46ada71b790eb526d6cddb91e0d0aed4cb

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