library: added PROCPS_PIDS_CGNAME for <pids> interface

library: added PROCPS_PIDS_CGNAME for <pids> interface

The ps program was modified to print the control group
names, based on the library provided list of all those
control groups to which a process belongs. But this is
probably something the newlib should be doing for all.

So this commit borrows the ps approach to cg names and
thus will make that available to all future consumers.

[ but stay tuned! there is a commit coming soon that ]
[ represents a rather major internal redesign, which ]
[ was prompted by the ps and top adaptation testing. ]

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