Display process slot number in the async overview 


Fix the number of column for 'Async connections'.
There are only 3 columns (writing, keep-alive, closing), not 4.

Try to improve the code layout for it to be more readable.
Each <th> is on its own line so keep the corresponding "colspan" <td> fields grouped together.

r1738628 introduced a new column, 'Slot'.
Add an empty cell for it in the last line of the table, in order to fix the layout of the Totals.

Replace tab by spaces to be consistent

mod_status: note stopping procs in async info table

* add new column "stopping", denoting if a process is shutting down
* add additional "(old gen)", if a process is from before a graceful reload
* add counts of processes and stopping processes to summary line


Submitted by: sf, jailletc36, jailletc36, sf
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1772333 13f79535-47bb-0310-9956-ffa450edef68