BUG/MEDIUM: conn-stream: Don't erase endpoint flags on reset

Only CS_EP_ERROR flag is now removed from the endpoint when a reset is
performed. When a new the endpoint is allocated, flags are preserved. It is
the caller responsibility to remove other flags, depending on its need.

Concretly, during a connection retry or a L7 retry, we must preserve
flags. In tcpcheck and the CLI, we reset flags.

This patch is 2.6-specific. No backport needed.
diff --git a/src/backend.c b/src/backend.c
index a887eb2..5a8bc0e 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1577,6 +1577,7 @@
 					srv_conn = NULL;
 					if (cs_reset_endp(s->csb) < 0)
 						return SF_ERR_INTERNAL;
+					s->csb->endp->flags &= CS_EP_DETACHED;
 				}
 			}
 			else