suEXEC: Add Suexec directive to disable suEXEC without renaming the

suEXEC: Add Suexec directive to disable suEXEC without renaming the
binary (Suexec Off), or force startup failure if suEXEC is required
but not supported (Suexec On).  Change SuexecUserGroup to fail
startup instead of just printing a warning if suEXEC is disabled.

Additionally, ap_unixd_config.suexec_disabled_reason has a message,
suitable for logging/messaging, explaining why the feature isn't
available.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1033519 13f79535-47bb-0310-9956-ffa450edef68