toollib: Rewrite process_each_vg.

Split VG argument collection from processing.
This allows the two different loops through VGs to
be replaced by a single loop.
Replace unused struct cmd_vg and cmd_vg_read() replicator
code with struct vg and vg_read() directly.

[Committed by agk with cosmetic changes and tweaks.]
toollib: Rewrite process_each_lv.

Copy the same form as the new process_each_vg.
Replace unused struct cmd_vg and cmd_vg_read() replicator
code with struct vg and vg_read() directly.
The failed_lvnames arg is no longer used since the
cmd_vg replicator wrapper was removed.

[Committed by agk with cosmetic changes and tweaks.]