libmultipath: iet prioritizer: support multiple subnets

Extend the prio_args for the iet prioritizer as follows:

   preferredip=<CIDR>:<Prio>,<CIDR>:<Prio>,...

where CIDR is an IPv4 subnet in CIDR format (o.o.o.o/p),
e.g 192.168.1.0/24. If the prefix is not given, it's assumed to
be 32. IP addresses can be abbreviated by ommitting trailing zeroes,
i.e. 10.2/16 is equivalent to 10.2.0.0/16. <Prio> is a numeric
priority to assign to paths to iSCSI servers whose IP address
is in the given range. If a a server IP address matches multiple
subnets, the one with the highest prefix takes precedence.

The default subnet 0.0.0.0/0 matches every IP address and has
priority 0 by default.

For backward compatibility, the syntax

   preferredip=<IP>

(without a prefix and priority) simply assigns an increased priority
to the given server IP address; it's equivalent to

  preferredip=<IP>/32:20,0.0.0.0/0:10

Author: Arnaldo Viegas de Lima