Fix a long delay with CGI requests and keepalive connections on

Fix a long delay with CGI requests and keepalive connections on
AIX.

On AIX, for processes like mod_cgid's script children where
SIGCHLD is ignored, kill(pid,0) returns success for up to
one second after the script child exits, based on when a
daemon runs to clean up unnecessary process table entries.
getpgid() can report the proper info (-1/ESRCH) immediately.

One user had a page with a lot of embedded images created by
CGIs, and the browser fetched them on a keepalive connection,
and the cumulative delays were very noticeable by the clients.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101716 13f79535-47bb-0310-9956-ffa450edef68