p.conf: add [global] proxy setting

if set, the http_proxy environment variable will be set to this value
by piuparts-master/-slave/-report (and will be propagated to piuparts)
overriding any http_proxy setting

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p: add --proxy option

retaining the http_proxy variable through sudo is difficult

switch from using HTTP_PROXY to http_proxy

Signed-off-by: Andreas Beckmann <anbe@debian.org>
Merge remote-tracking branch 'andreas/feature/proxy' into develop