RM6541 ip_availability script: fix unused count

Removed deallocated, reusable IP count from unused count because the
number of deallocated, reusable IPs was already included as part of
the subnet's size.
RM6541: ip_availabilty script: fixes _cidr filter

Replaced _cidr filter on IPv6 addresses and 10.% addresses by
checking for ip_version = 4 and network_id = all 0's.
Merge pull request #134 from asadoughi/master

RM6541: ip_availability script minor fixes