Change the default setting of UseCanonicalName to off and remove

Change the default setting of UseCanonicalName to off and remove
that directive from the default config.

This could use some review because it has potential security
implications.  In particular, it could cause cross-site scripting
vulnerabilities if people rely on SERVER_NAME without validating
it.  But we have been running with this in our default config file
for a while, and it vastly reduces the confusion that comes
from people setting ServerName incorrectly.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/simple-conf@160595 13f79535-47bb-0310-9956-ffa450edef68