Add privileged port bind capabilities for Linux

This adds the support for Tor to bind to privileged port for an
unprivileged user using the Linux capabilities(7).

To achieve such a thing, an option called UseCapabilities is added to
the torrc and allow the user to use that or not. By default it's set to
0 (off).

This uses libcap2 API if found on the system else it works without it by
ignoring the option and not trying to use capabilities.

Fixes #8195

Signed-off-by: David Goulet <dgoulet@ev0ke.net>