Introduce modules/system, and mod_unixd.c

Introduce modules/system, and mod_unixd.c

mod_unixd.c is only os/unix/unixd.c, turned into a module.
(or more accurately, an ugly hack).  Work in progress, and the
old unixd will have to go once the module build stuff is in place.

This re-introduces some of the unixd stuff (set user, group and chroot)
to mpm_simple, and should work equally well with existing MPMs.
It also paves the way for alternative privilege management schemes,
which can be implemented in alternative modules: this´ll be
necessary to run mpm_simple on non-unix-family platforms, and
of interest for alternative Unix implementations.  By relegating
everything to a child_init hook, we can mix-and-match, though
weĺl need to add a regime that prevents just going right ahead
with nothing and running as root.

Round tuits permitting, we should be able to simplify unixd.h too,
and look towards eliminating it.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@708935 13f79535-47bb-0310-9956-ffa450edef68