service: Add an option to enable debugging

• Add and document a -d option which enables rc script debugging.
• Sort and document existing options correctly.

MFC after: 1 week
Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D51700

(cherry picked from commit 25fca243f2b7ca0c523aa1cf26aaea25035e68d2)