bind: security bump to version 9.6-ESV-R4

Bump bind to version 9.6-ESV-R4 since the 9.5 series is no longer
supported.
While at it switch to the ESV (Extended Support Version) branch instead
of going for the latest.

Also adjust the package to build against OpenSSL when available for
proper DNSSEC support and libxml2 too.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>