library: add real string vectors to the pids interface

library: add real string vectors to the pids interface

After experimenting with an adaptation of pidof to the
new pids interface, it became apparent that vectorized
versions of those command lines would be necessary. So
this commit adds that option and the strv result type.

And since the stage had been set, a vectorized version
of PROCPS_PIDS_ENVIRON & PROCPS_PIDS_CGROUP was added.

Lastly, any use of 'const' in the result structure was
removed so callers need not be bothered with casts and
compiler warnings. Hopefully, they'll respect a stack.

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