local-unbound: Read a tab separated resolv.conf

Use :space: rather than a white space character to delimit the keys
and the values in the resolv.conf file.

PR: 236079
Reviewed by: des
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53811

(cherry picked from commit 0628400590e025b7db1c0905e6ee488a24ef3f60)