Replace PV_MIN_SIZE with function pv_min_size()

Replace PV_MIN_SIZE with function pv_min_size()

Add configurable option to define minimal size of
of block device usable as a PV.

pv_min_size() is added to lvm-globals and it's being
initialized through _process_config.

Macro PV_MIN_SIZE is unused and removed.

New define DEFAULT_PV_MIN_SIZE_KB is added to lvm-global
and unlike PV_MIN_SIZE it uses KB units.

Should help users with various slow devices attached to the system,
which cannot be easily filtered out (like FDD on /dev/sdX):
https://bugzilla.redhat.com/show_bug.cgi?id=644578