check address family consistency in generator

check address family consistency in generator

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.