add option for selective PTR generation on hosts.cgi
In some cases, it might be useful to create an additional
host (i.e. for round robin loadbalancing) without assigning
another PTR to the IP address specified.
This patch introduces the ability to check or uncheck
PTR generation for each host individually.
Partially fixes #12030
Signed-off-by: Peter Müller <peter.mueller@ipfire.org> Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>