Add support for specifying source IPs for outgoing connections

[defanator]

Introduced the -b (or --bind-ip) command line option, that can accept either single IP address, or CIDR mask, e.g.:

wrk -b 127.0.0.2 http://localhost/
wrk -b 127.0.0.1/28 http://localhost/

In addition, the IP_BIND_ADDRESS_NO_PORT socket option is being used on supported systems (Linux >= 4.2, glibc >= 2.23) in order to get even more concurrent connections due to an ability to share source port (see [1], [2]).

Also, EADDRNOTAVAIL from connect() is now treated as a critical error, leading wrk to quit immediately.

[1] https://kernelnewbies.org/Linux_4.2#head-8ccffc90738ffcb0c20caa96bae6799694b8ba3a [2] https://git.kernel.org/torvalds/c/90c337da1524863838658078ec34241f45d8394d