MEDIUM: httpclient: httpclient_create_proxy() creates a proxy for httpclient

MEDIUM: httpclient: httpclient_create_proxy() creates a proxy for httpclient

httpclient_create_proxy() is a function which creates a proxy that could
be used for the httpclient. It will allocate a proxy, a raw server and
an ssl server.

This patch moves most of the code from httpclient_precheck() into a
generic function httpclient_create_proxy().

The proxy will have the PR_CAP_HTTPCLIENT capability.

This could be used for specifics httpclient instances that needs
different proxy settings.