useragent: Automatically choose whether to use LWPx::ParanoidAgent

useragent: Automatically choose whether to use LWPx::ParanoidAgent

The simple implementation of this, which I'd prefer to use, would be:
if we can import LWPx::ParanoidAgent, use it; otherwise, use
LWP::UserAgent.

However, aggregate has historically worked with proxies, and
LWPx::ParanoidAgent quite reasonably refuses to work with proxies
(because it can't know whether those proxies are going to do the same
filtering that LWPx::ParanoidAgent would).

Signed-off-by: Simon McVittie <smcv@debian.org>