library: implement task/thread support via the new api

library: implement task/thread support via the new api

This commit is the culmination of efforts to modernize
the library api. It should be treated as a first blush
attempt, especially since I have absolutely no library
design experience. But I did have a very strong desire
to lessen the new library's impact on the top program.

Under this new api, a 'stack' is the equivalent of the
old proc_t. It can be seen as a variable length record
whose contents & order is under complete user control.

That initial stack/record configuration is established
at procps_pids_new() time and will probably serve most
program needs. But, a dynamic & demanding program like
top will later change a stack via procps_pids_reset().

For programs like top & ps, procps_pids_reap() will be
the function that will retrieve all tasks and threads.

Any program that needs to filter / select only certain
processes or users have available other functions that
can be used: procps_pids_stacks_alloc, fill & dealloc.

This implementation attempts to maximize that existing
proven libprocps code base. As we gain more experience
such actual code can be migrated into the pids.c file.

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