git-instaweb: support Fedora/Red Hat apache module path

git-instaweb: support Fedora/Red Hat apache module path

On Fedora-derived systems, the apache httpd package installs modules
under /usr/lib{,64}/httpd/modules, depending on whether the system is
32- or 64-bit.  A symlink from /etc/httpd/modules is created which
points to the proper module path.  Use it to support apache on Fedora,
CentOS, and Red Hat systems.

Written with assistance of Todd Zullinger <tmz@pobox.com> and
Junio C Hamano <gitster@pobox.com>.

Signed-off-by: Sebastian Kisela <skisela@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>