depend-cleanup: Add verbose (-v) and pretend/dry-run (-n) flags

These are useful for testing new additions to the script. Whilst here,
harden the script a little and improve error messages.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D31007

(cherry picked from commit 1ec7cb1b5d1800b05c0682e0b673be0dece98779)

Pull Request: https://github.com/freebsd/freebsd-src/pull/1543