BUG/MINOR: httpclient: reinit flags in httpclient_start()

BUG/MINOR: httpclient: reinit flags in httpclient_start()

When starting for the 2nd time a request from the same httpclient *hc
context, the flags are not reinitialized and the httpclient will stop
after the first call to the IO handler, because the END flag is always
present.

This patch also add a test before httpclient_start() to ensure we don't
start a client already started.

Must be backported in 2.5.

(cherry picked from commit 5085bc3103e5a52f0d8909d13a6e36d24d2a6562)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>