process_each_lv: add check_single_lv function

process_each_lv: add check_single_lv function

The new check_single_lv() function is called prior to the
existing process_single_lv().  If the check function returns 0,
the LV will not be processed.

The check_single_lv function is meant to be a standard method
to validate the combination of specific command + specific LV,
and decide if the combination is allowed.  The check_single
function can be used by anything that calls process_each_lv.

As commands are migrated to take advantage of command
definitions, each command definition gets its own entry
point which calls process_each for itself, passing a
pair of check_single/process_single functions which can
be specific to the narrowly defined command def.