library: allow negative delta values for 2, <STAT> api

library: allow negative delta values for 2, <STAT> api

A recent buglet in the <meminfo> interface, where many
delta values could legitimately be negative, suggested
a review of this module where a minus was forced to 0.

As it turns out, there are two delta items that indeed
could be negative. They involve the current processes.

So henceforth, DELTA_PROC_BLOCKED & DELTA_PROC_RUNNING
will now be allowed to go negative. I believe that all
other items can only grow. But, if we find more later,
at least the adaptation approach has been established.

[ this whole business of checking for less than zero ]
[ harks back to an old kernel anomaly where negative ]
[ tics were sometimes experienced. top was affected. ]

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