Follow on to r1393976 and r1396424:

mod_systemd: Specify "all" instead of "yes" (on Unix) so that APACHE_MODULE doesn't
             act as if the user requested it.

General: Fix APACHE_MODULE so that failed prereqs will disable a module unless the
         user requested it explicitly.

Result: By default systemd will be enabled where it actually works and disabled otherwise,
        but if the user requests it and the prereqs fail then configure will halt.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1396440 13f79535-47bb-0310-9956-ffa450edef68