MFC r312499:

Use SRCTOP-relative paths to other directories instead of .CURDIR-relative ones

This simplifies pathing in make/displayed output

MFH (r266114, r266138): upgrade to latest ldns and unbound
MFH (r266139-r266143, r266145, r266149, r266150): fix props
MFH (r266179, r266180, r266193, r266238, r266777): misc cleanup
MFH (r266863): create and use /var/unbound/conf.d
MFH (r268839): import unblock-lan-zones patch from upstream
MFH (r268840): fix reverse lookups on private networks
MFH (r268883): avoid spamming source tree during build

PR: 190739 (for r268883)

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

LDNS needs OpenSSL. This wasn't a problem as long as it was only build
statically, since any program using it would have to link with it anyway.

Approved by: re (blanket)

Make libldns and libssh private.

Approved by: re (blanket)

Clean up.

Include ldns's b{32,64}_{ntop,pton}().

Upgrade to 1.6.16

Import LDNS and build it as an internal library.