apache: revisit suEXEC setup

apache: revisit suEXEC setup

When adding suEXEC to the apache package, Alpine's package [1] served as
a template. Not enough attention was paid to the details.

Alpine uses a different layout. So for OpenWrt to use /var/www as
DocumentRoot does not make sense. /var is also volatile on OpenWrt. This
commit removes the configure argument. The default is htdocsdir.

This also does away with uidmin/gidmin 99. The default is 100, which is
fine.

Finally, the suexec binary is moved from /usr/sbin to
/usr/lib/apache2/suexec_dir. Upstream recommends installing suexec with
"4750" (see [2]) and the group set to the user's group. While that would
be possible, it would cause a few headaches on OpenWrt. The group would
need to be changed first in a post-install script and a call to chmod
would need to be made afterward, to make the binary SUID again.

It's easier to hide the SUID binary away from others in a directory.
This way we don't need to use chmod in the post-install script.

[1] https://github.com/alpinelinux/aports/tree/master/main/apache2
[2] https://httpd.apache.org/docs/2.4/suexec.html

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>