Let SIGALRM do its thing even if we're on the right millisecond.

Let SIGALRM do its thing even if we're on the right millisecond.

If we request a timer interrupt now in logic that closes a lot of
files, which runs through %deregister-io-descriptor, it looks like
_all_ the time is spent in closing files even if it's merely the case
that at least one file is closed every millisecond.

If we instead let SIGALRM do its thing, unless the interrupt is
_overdue_, then the profiler can discriminate at sub-millisecond
resolution where the time is spent which is what I really want.

(cherry picked from commit 6748a3cf190f5232d25483f15a833e8a61e706be)