MINOR: task: add an application specific flag to the state: TASK_F_USR1

MINOR: task: add an application specific flag to the state: TASK_F_USR1

This flag will be usable by any application. It will be preserved across
wakeups so the application can use it to do various stuff. Some I/O
handlers will soon benefit from this.

(cherry picked from commit 6fa8bcdc785c29cd3a29c14d2038f38b5547eff0)
[wt: needed for upcoming fixes. task->state is 16-bit in 2.3 and before
 so we've reused bit 15 which was still available there. Ctx adjusments
 since no TASK_F_TASKLET here]
Signed-off-by: Willy Tarreau <w@1wt.eu>