On graceless shutdown or restart, send AP_SIG_GRACEFUL to all worker

On graceless shutdown or restart, send AP_SIG_GRACEFUL to all worker
threads so that any that are polling on Keep-Alive connections will
wake up and exit promptly.  Otherwise, on Linux, they wait until
poll()'s timeout interval completes, which is often too long for
the parent process; the parent issues SIGKILL before the child's main
thread manages to re-join all the worker threads.  PR 38737.

On Solaris, by contrast, the close_worker_sockets() function
successfully alerts all worker threads that they should exit quickly.
On Linux, this side-effect of one thread closing another's socket
doesn't occur; see:

http://bugme.osdl.org/show_bug.cgi?id=546

Also, expand on the placeholder in CHANGES regarding previous
cleanups to scoreboard.c and scoreboard.h in r404848 and r404849.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@409715 13f79535-47bb-0310-9956-ffa450edef68