strace: further optimize unblocking of the delay signal handler

strace: further optimize unblocking of the delay signal handler

Do not unblock the delay signal handler unless the delay timer is armed.

* defs.h (is_delay_timer_created): Remove.
(is_delay_timer_armed, delay_timer_expired): New prototypes.
* delay.c (delay_timer_is_armed): New static variable.
(is_delay_timer_created): Add static qualifier.
(is_delay_timer_armed, delay_timer_expired): New functions.
(arm_delay_timer): Set delay_timer_is_armed.
* strace.c (next_event): Use is_delay_timer_armed instead of
is_delay_timer_created to check whether the delay signal handler
has to be unblocked.
(timer_sighandler): Invoke delay_timer_expired.