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.

(cherry picked from commit 519abefb57da1ae21fc557213cae8b21cdaa2797)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 008045a03eeca63712546e030a4549671d00ffc1)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit cba2b004accf766a3c6e6aac35852e27cd74cf07)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>