framework: new hook to only allow 'default' files in category/port

This hooks enforces that all files in the top-directory of a port match:

• .*\.mk
• Makefile.*
• distinfo.*
• pkg-.*

An example error message would look like:

[pre-commit] ERROR: invalid file 'Foo' in 'kate/editors'

Consider moving non-standard files to files/ or force-ignore this hook.

Differential Revision: https://reviews.freebsd.org/D37387