Add documentation for access control system

Add documentation for access control system

This adds two new pages to the website, acl.html describing
the general access control framework and permissions models,
and aclpolkit.html describing the use of polkit as an
access control driver.

page.xsl is modified to support a new syntax

  <div id="include" filename="somefile.htmlinc"/>

which will cause the XSL transform to replace that <div>
with the contents of 'somefile.htmlinc'. We use this in
the acl.html.in file, to pull the table of permissions
for each libvirt object. This table is autogenerated
from the enums in src/access/viraccessperms.h by the
genaclperms.pl script.

newapi.xsl is modified so that the list of permissions
checks shown against each API will link to the description
of the permissions in acl.html

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>