MEDIUM: stconn: Be sure to always be able to unblock a SC that needs room

MEDIUM: stconn: Be sure to always be able to unblock a SC that needs room

When sc_need_room() is called, the caller cannot request more free space
than a minimum value to be sure it is always possible to unblock it. it is a
safety guard to not freeze any SC on NEED_ROOM condition. At worse it will
lead to some wakeups un excess at the edge.

To keep things simple, the following minimum is used:

  (global.tune.bufsize - global.tune.maxrewrite - sizeof(struct htx))