Configuration: Introduce $c{Username}

Configuration: Introduce $c{Username}

This makes it easier to share the output of whoami.  As a beneficial
side effect it can now be overridden.

Replace many open-coded calls to `whoami` etc. with references to
$c{Username}.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
---
v2: New patch