LP#1476049: disable serve-cgi-bin Apache config on Jessie and Xenial

LP#1476049: disable serve-cgi-bin Apache config on Jessie and Xenial

This patch ensures that the stock Apache configuration
"serve-cgi-bin" is disabled when installing on Debian Jessie
and Ubuntu Xenial, as otherwise the default cgi-bin location
can override what Evergreen specifies for offline.pl.

To test:

[1] Run the Makefile.install step on Jessie or Xenial; verify
    that there is no /etc/apache2/conf-enabled/serve-cgi-bin.conf
    symlink.
[2] Proceed with the rest of the Evergreen installation; verify
    that the Apache error log doesn't contain a warning like this:

    "The Alias directive in /etc/apache2/sites-enabled/eg.conf ...
     will probably never match because it overlaps an earlier
     ScriptAlias."

[3] For extra credit, verify that one can access offline circulation
    sessions.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>