mpm_unix(es): cleanup properly on exit in one_process mode.

We can't destroy ap_pglobal in the MPMs because clean_child_exit() runs in
a DSO which would be unloaded under us.

So we defer an ap_terminate() with atexit() in ap_unixd_mpm_set_signals(),
all this is static/builtin code in "os/unix/unixd.c".


Follow up to r1822537: replace static variable with pool userdata.

Also adds a comment and a CHANGES entry.


Submitted by: ylavic
Reviewed by: ylavic, jim, icing


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1833005 13f79535-47bb-0310-9956-ffa450edef68