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). Also, all
capabilities will be dropped except the ones Tor need.

To achieve such a thing, an option called UseCapabilities is added to
the torrc and allow the user to use it 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>