exmaples: add --verify option in the query example

[keepsimple1]

Address the comments in #54 .

Now the query example supports --verify option (doc) to show how it can be used for reconfirming service IPv4 addresses:

$ cargo run --example query   
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/examples/query`
Usage: cargo run --example query <service_type_without_domain_postfix> [--verify]
Example: 
cargo run --example query _my-service._udp

Options:
--verify: make the client attempt to verify IPv4 addresses of resolved services.

You can also do a meta-query per RFC 6763 to find which services are available:
cargo run --example query _services._dns-sd._udp

Note that unregister already exists in the register example:

$ cargo run --example register
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s
     Running `target/debug/examples/register`
Usage:
cargo run --example register <service_type> <instance_name> <hostname> [options]

Options:

--unregister: automatically unregister after 2 seconds
--disable-ipv6: not to use IPv6 interfaces.
--logfile: write debug log to a file instead of stderr. The logfile is named 'mdns-register-<timestamp>.log'.

For example:
cargo run --example register _my-hello._udp instance1 host1

To see the debug log, set the RUST_LOG environment variable and write a logfile:
RUST_LOG=mdns_sd=debug cargo run --example register _my-hello._udp instance1 host1 --logfile