MEDIUM: httpclient: allow to use another proxy

httpclient_new_from_proxy() is a variant of httpclient_new() which
allows to create the requests from a different proxy.

The proxy and its 2 servers are now stored in the httpclient structure.

The proxy must have been created with httpclient_create_proxy() to be
used.

The httpclient_postcheck() callback will finish the initialization of
all proxies created with PR_CAP_HTTPCLIENT.
3 files changed