Add configuration option for the LDNS DNSSEC Anchor file.

[manonfgoo]

The DNSSEC Anchor required for VerifyHostKeyDNS can't always be easily set in /etc/resolv.conf. DHCP and other tasks may overwrite this file or the user may want to use the DNSSEC Anchor different from what is configured in /etc/resolv.conf. This patch allows to set the DNSSEC Anchor File with the option LDNSAnchorFile.

Example: ssh -oLDNSAnchorFile=/etc/unbound/root.key -oVerifyHostKeyDNS=yes example.com