git-instaweb: fix apache2 config with apache >= 2.4

git-instaweb: fix apache2 config with apache >= 2.4

The generated apache2 config fails with apache >= 2.4.  The error log
states:

    AH00136: Server MUST relinquish startup privileges before accepting
    connections.  Please ensure mod_unixd or other system security
    module is loaded.
    AH00016: Configuration Failed

Fix this by loading the unixd module.  This works with older httpd as
well, so no IfVersion conditional is needed.  (Tested with httpd-2.2.15
on CentOS-6.)

Written with assistance of Todd Zullinger <tmz@pobox.com>

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