MFC r304652:

Fix missing substitution of @SBINDIR@ in resolvconf scripts

Certain features, such as resolv_conf_passthrough=NULL, do not work
correctly due to this missing substitution.

Also remove the @PREFIX@ substitution, which is no longer needed.

Approved by: vangyzen (mentor)
Sponsored by: Dell Inc.

MFC r282434:
MFV r225523, r282431:
r225523 (hrs):
Import openresolv-3.4.4.

r282431:
Import openresolv-3.7.0.

PR: 199854
Submitted by: yuri@rawbw.com
Relnotes: yes
Sponsored by: The FreeBSD Foundation

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

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

Add resolvconf(8) which manages resolv.conf.