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.

Reviewed by: pfg
Approved by: vangyzen (mentor)
MFC after: 1 week
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D7572