Fix PR36410; Change how the get_suexec_identity hook is handled by CGID.

Fix PR36410; Change how the get_suexec_identity hook is handled by CGID.
Instead of using mod_userdir and mod_suexec specific hacks, we now run the hook
on the httpd side of the handler.

If this is NULL, we pass on a magic empty_ugid constant, otherwise pass on the
real ugid.

On the cgid side of the equation, we add our own hook, with REALLY_FIRST, and
then order the hooks. This ensures that cgid's doer runs before any other
registered get_suexec_identity doers.

We use cgid's request config to store the ugid. If ugid == empty_ugid, we DON'T
call ap_os_create_privileged_process, because our doer would return the magic
empty_ugid constant. Having the doer return NULL is no good, because then
userdir and mod_suexec's doers would be called. Instead, we call plain old
apr_proc_create().

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@264759 13f79535-47bb-0310-9956-ffa450edef68