Before, "fping -6 -g IPv4_ADDR_SPEC" would generate all requested
IPv4 addresses and try to add them to the host table, but that
would fail because of address family mismatch.

Now, "fping -6 -g IPv4_ADDR_SPEC" detects the problem before
generating any addresses, and fails early.

Mixing IPv4 and IPv6 addresses when using -g START END is detected
before generating any addresses and fails early, too.

This creates a code structure suitable to add IPv6 versions
of the functions used to generate addresses.

Similarly to IPv4, using CIDR notation excludes a special
per-subnet address, the Subnet-Router anycast address.

The limit for generated addresses is increased by one to
allow /111 IPv6 prefixes similar to /15 IPv4 prefixes.

The implementation uses two unsigned 64 bit integers to
represent a single IPv6 address.  The required arithmetic
is open-coded and limited to what is needed to implement
IPv6 support for -g, --generate.

This addresses GitHub issue schweikert#59.