ports.7: Add port files to FILES

Add terse descriptions for the individual files that make up a port to
the FILES section of the ports reference manual.

MFC after: 3 days
Reviewed by: adamw
Differential Revision: https://reviews.freebsd.org/D55404

(cherry picked from commit 451f593194cbea74bcae53b4b8e6e55f6b2ec12d)