MEDIUM: init: always warn when running as root without being asked to

MEDIUM: init: always warn when running as root without being asked to

Like many exposed network deamons, haproxy does normally not need to run
as root and strongly recommends against this, unless strictly necessary.
On some operating systems, capabilities even totally alleviate this need.

Lately, maybe due to a raise of containerization or automated config
generation or a bit of both, we've observed a resurgence of this bad
practice, possibly due to the fact that users are just not aware of the
conditions they're using their daemon.

Let's add a warning at boot when starting as root without having requested
it using "uid" or "user". And take this opportunity for warning the user
about the existence of capabilities when supported, and encouraging the
use of a chroot.

This is achieved by leaving global.uid set to -1 by default, allowing us
to detect if it was explicitly set or not.