libfetch: Clean up fetch_info usage

• Provide a wrapper for the common if (verbose) fetch_info(...) idiom.

• Replace remaining instances of fprintf(stderr, ...) with fetch_info().

• Fix a few style nits.

MFC after: 1 week
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D55405

(cherry picked from commit 1b7c4d29fdfc975facdc43f6da50947cf3bb8e4a)