ports.7: Document DEBUG_FLAGS and the process of debugging ports

Loosely based on https://wiki.freebsd.org/BenWoods/DebuggingPorts.

Reviewed by: crees
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D27296

(cherry picked from commit d25f7d324a9d0c33655876b920ba590918d16f4c)