Clarify/Improve the http_client proxy settings behaviour

[frispete]

Hi,

in a customer setup, we experienced a somewhat unexpected behaviour:

That site policy required using a (transparent) proxy. We noticed, that changing the system proxy using yast proxy didn't work as expected (no effect). After a while, we noticed a deviating proxy setting in /etc/rmt.conf, and @digitaltom commented, that rmt ignores the system proxy setting altogether.

After thinking a bit more about this issue, I would like to recommend this change (from higher to lower priority):

• If a proxy setting is found in /etc/rmt.conf, use it, but also define a way to forcefully disable the proxy setting here (e.g.: http_client.proxy: None)
• If a system proxy setting is defined, use that

Print a message on setup, which proxy setting is in use (if there is one..).
Add a note to the documentation/man page about this behaviour (http_client.proxy vs. http_proxy environment).
While at it, the man page seems to be missing documentation of some possible settings in the http_client section.

If the proxy logic is implemented as such, we cover all kinds of scenarios, including some esoteric ones.
Specifically allow the system proxy to differ from the rmt proxy, while we still adhere to the principle of least astonishment (POLA).