library: privatize two with <pids> select/fill changes

library: privatize two with <pids> select/fill changes

After simplifying that select/fill interface, there is
no longer a need for public 'alloc' & 'dealloc' stacks
functions. There is now only one instance of stacks as
an input parameter found in procps_pids_stacks_sort().
But sorting 'empty' stacks serves no possible purpose.

So this commit retains both functions, since they will
still be needed, but designates them private (static).

Additionally, with their demise we will eliminate that
pids_stacks structure from the header file, internally
using what always was the true 'stacks_extent' struct.

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