BUG/MINOR: httpclient: wrongly named httpproxy flag

BUG/MINOR: httpclient: wrongly named httpproxy flag

The HC_F_HTTPPROXY flag was wrongly named and does not use the correct
value, indeed this flag was meant to be used for the httpclient API, not
the httpclient CLI.

This patch fixes the problem by introducing HTTPCLIENT_FO_HTTPPROXY
which has must be set in hc->flags.

Also add a member 'options' in the httpclient structure, because the
member flags is reinitialized when starting.

Must be backported as far as 3.0.