[apps] Added setopt utility for easily setting socket options

[ethouris]

This adds a friendly C++ wrapper over the srt_setsockflag call:

srt::setopt(sock)[OPTION] = value;

instead of

type value = something;
srt_setsockflag(sock, OPTION, &value, sizeof value);

Also an example usage was added to srt-test-mpbond app.