liblvm: allow config settings to be read without full lvm cmd

liblvm: allow config settings to be read without full lvm cmd

A program may be using liblvm2app for simply checking a config
setting in lvm.conf.  In this case, a full lvm context is not
needed, only cmd->cft (which are the config settings read from
lvm.conf).

lvm_config_find_bool() can now be passed a NULL lvm context
in which case it will only create cmd->cft, check the config
setting asked for, and destroy the cmd.